The Vulnerability is the bait
Within Star Wars Episode VI: Return of the Jedi, The Emperor reveals his entire plan to Luke, which also serves as the answer to your question.
This entire conceit of the Emperor’s plan is served from the film’s opening moment, convincing not only the characters of the film, but also trick the audience as well:
JERJERROD: Lord Vader, this is an unexpected pleasure. We're honored by your presence.
VADER: You may dispense with the pleasantries, Commander. I'm here to put you back on schedule.
JERJERROD: I assure you, Lord Vader, my men are working as fast as they can.
VADER: Perhaps I can find new ways to motivate them.
JERJERROD: I tell you, this station will be operational as planned.
VADER: The Emperor does not share your optimistic appraisal of the situation.
JERJERROD: But he asks the impossible. I need more men.
VADER: Then perhaps you can tell him when he arrives.
JERJERROD: The Emperor's coming here?
VADER: That is correct, Commander. And he is most displeased with your apparent lack of progress.
JERJERROD: We shall double our efforts.
VADER: I hope so, Commander, for your sake. The Emperor is not as forgiving as I am.
This conceit then gets slow rolled through the rest of the film, foreshadowing the Emperor’s plan and setting the hooks of his planned traps.
VADER: The Death Star will be completed on schedule.
EMPEROR: You have done well, Lord Vader. And now I sense you wish to continue your search for young Skywalker.
VADER: Yes, my Master.
EMPEROR: Patience, my friend. In time he will seek you out. And when he does, you must bring him before me. He has grown strong. Only together can we turn him to the dark side of the Force.
VADER: As you wish.
EMPEROR: Everything is proceeding as I have foreseen.
We learn the Emperor’s planning has been successful, as we learn about the Rebel Alliance:
- The Rebel Fleet will attack the Death Star;
- A Rebel Strike team will attack the deflector shield;
- Luke will be a part of it.
Here, your answer is first revealed by Ackbar in his description of the Rebel plan.
The vulnerability is the key (bait):
MON MOTHMA: The Emperor has made a critical error and the time for our attack has come.
MON MOTHMA: The data brought to us by the Bothan spies pinpoints the exact location of the Emperor's new battle station. We also know that the weapon systems of this Death Star are not yet operational. With the Imperial Fleet spread throughout the galaxy in a vain effort to engage us, it is relatively unprotected. But most important of all, we've learned that the Emperor himself is personally overseeing the final stages of the construction of this Death Star.
MON MOTHA: Many Bothans died to bring us this information. Admiral Ackbar, please.
ACKBAR: You can see here the Death Star orbiting the forest Moon of Endor. Although the weapon systems on this Death Star are not yet operational, the Death Star does have a strong defense mechanism. It is protected by an energy shield, which is generated from the nearby forest Moon of Endor. The shield must be deactivated if any attack is to be attempted. Once the shield is down, our cruisers will create a perimeter, while the fighters fly into the superstructure and attempt to knock out the main reactor.
ACKBAR: General Calrissian has volunteered to lead the fighter attack.
HAN: Good luck.
HAN: You're gonna need it.
ACKBAR: General Madine.
GENERAL MADINE: We have stolen a small Imperial shuttle. Disguised as a cargo ship, and using a secret Imperial code, a strike team will land on the moon and deactivate the shield generator.
THREEPIO: Sounds dangerous.
LEIA: I wonder who they found to pull that off.
GENERAL MADINE: General Solo, is your strike team assembled?
HAN: Uh, my team's ready. I don't have a command crew for the shuttle.
HAN: Well, it's gonna be rough, pal. I didn't want to speak for you.
HAN: That's one.
LEIA: Uh, General... count me in.
VOICE: I'm with you, too!
After the Rebels put their plan in motion, and the path to the film’s conflict locked in, the Emperor finally begins to reveal there is more going on that the eye has seen; both to his servants as well as for us:
VADER: What is thy bidding, my Master?
EMPEROR: Send the fleet to the far side of Endor. There it will stay until called for.
VADER: What of the reports of the Rebel fleet massing near Sullust?
EMPEROR: It is of no concern. Soon the Rebellion will be crushed and young Skywalker will be one of us! Your work here is finished, my friend. Go out to the command ship and await my orders.
VADER: Yes, my Master.
The Imperials continue to put the Emperor’s plans into motion as the Rebels confirm they’ve swallowed his bait:
VADER: Where is that shuttle going?
PIETT: Shuttle Tydirium, what is your cargo and destination?
(HAN): Parts and technical crew for the forest moon.
VADER: Do they have a code clearance?
PIETT: It's an older code, sir, but it checks out. I was about to clear them.
PIETT: Shall I hold them?
VADER: No. Leave them to me. I will deal with them myself.
PIETT: As you wish, my lord. Carry on…
HAN: They're not goin' for it, Chewie.
CONTROLLER: Shuttle Tydirium, deactivation of the shield will commence immediately. Follow your present course.
HAN: Okay! I told you it was gonna work. No problem.
We learn that even as the characters play out according to their own interests and desires, it all still serves the Emperor’s trap:
EMPEROR: I told you to remain on the command ship.
VADER: A small Rebel force has penetrated the shield and landed on Endor.
EMPEROR: Yes, I know.
VADER: My son is with them.
EMPEROR: Are you sure?
VADER: I have felt him, my Master.
EMPEROR: Strange, that I have not. I wonder if your feelings on this matter are clear, Lord Vader.
VADER: They are clear, my Master.
EMPEROR: Then you must go to the Sanctuary Moon and wait for them.
VADER: He will come to me?
EMPEROR: I have foreseen it. His compassion for you will be his undoing. He will come to you and then you will bring him before me.
VADER: As you wish.
The Emperor has planned the entirety of the RotJ climax, including luring Rebels to Endor, the Rebel Fleet to the Second Death Star, and Luke to the Emperor. He first makes Luke feel powerless to stop him.
EMPEROR: Welcome, young Skywalker. I have been expecting you.
EMPEROR: You no longer need those.
EMPEROR: Guards, leave us.
And then once this has been done, he first gets some Sithly business out of the way, as additional dog-piling on the sense of helplessness, twisting the knife on Luke’s father.
EMPEROR: I'm looking forward to completing your training. In time you will call me Master.
LUKE: You're gravely mistaken. You won't convert me as you did my father.
EMPEROR: Oh, no, my young Jedi. You will find that it is you who are mistaken...about a great many things.
VADER: His lightsaber.
EMPEROR: Ah, yes, a Jedi's weapon. Much like your father's. By now you must know your father can never be turned from the dark side. So will it be with you.
LUKE: You're wrong. Soon I'll be dead...and you with me.
The Emperor laughs at him, because the Emperor thinks he knows more than Luke does about what is happening.
EMPEROR: Perhaps you refer to the imminent attack of your Rebel fleet.
EMPEROR: Yes...I assure you we are quite safe from your friends here.
LUKE: Your overconfidence is your weakness.
EMPEROR: Your faith in your friends is yours.
VADER: It is pointless to resist, my son.
EMPEROR: Everything that has transpired has done so according to my design. Your friends up there on the Sanctuary Moon... are walking into a trap. As is your Rebel fleet! It was I who allowed the Alliance to know the location of the shield generator. It is quite safe from your pitiful little band. An entire legion of my best troops awaits them.
EMPEROR: Oh...I'm afraid the deflector shield will be quite operational when your friends arrive.
So, here the Emperor was intending to stop the commando force on the moon, the Rebel Fleet sent to kill the Emperor and destroy the Death Star, and turn Luke.
EMPEROR: Come, boy. See for yourself.
EMPEROR: From here you will witness the final destruction of the Alliance, and the end of your insignificant Rebellion.
EMPEROR: You want this, don't you? The hate is swelling in you now. Take your Jedi weapon. Use it. I am unarmed. Strike me down with it. Give in to your anger. With each passing moment, you make yourself more my servant.
EMPEROR: It is unavoidable. It is your destiny. You, like your father, are now mine!
The Emperor had cast a small net of those needing to know, but Imperial leadership was sort of in on the plan:
COMMANDER: We're in attack position now, sir.
PIETT: Hold here.
COMMANDER: We're not going to attack?
PIETT: I have my orders from the Emperor himself. He has something special planned for them. We only need to keep them from escaping.
The Rebels had to believe they had a chance to destroy the Death Star with the Empire’s leadership with it. The point of the Death Star in RotJ was to serve as sacrificial bait in the Emperor’s plan to destroy the Rebel Alliance once and for all.