Why was Finn on Jakku?

Question

In the beginning of Star Wars: The Force Awakens, we meet Finn

as a member of a raiding party of Stormtroopers that attacked and destroyed a village on Jakku, where Poe and BB-8 were obtaining a missing fragment of a map that would lead the Resistance to find Luke Skywalker.

Finn experienced an act of conscience due to that event.

Not agreeing with what the First Order did, he left his position, first trying to escape to the Outer Rim, then later defecting to the Resistance.

Towards the end of the film, Finn reveals that

he worked in sanitation while stationed at Starkiller Base, and that the village attack was his first real field combat.

So, my question is - how did Finn go from that position to field combat in a major operation to retrieve vital information, without seemingly doing anything in between? It seems too convenient for me, just a way to make sure he was involved in two key points of the story:

the first being the event that caused BB-8 to go on the run and eventually met up with Rey and later Finn. And the other event where Finn happened to know a way into Starkiller Base and knew someone who they could force to lower the shields protecting the super weapon.

Answer 1

"So, my question is - how did Finn go from that position to field combat in a major operation to retrieve vital information, without seemingly doing anything in between? It seems too convenient for me, just a way to make sure he was involved in two key points of the story:"

This is covered (and partially implied) in a prequel book "Before the Awakening".

• As shown in the story, Finn, basically, was an exemplary cadet if you judge his combat proficiency. He was great at arms, and melee combat.

• Being on a sanitation detail likely wasn't a reflection of him as a soldier, but of him being a useless cay-dette at the time he was on Starkiller. SOMEONE's gotta dig latrines, and green trainees are just the right set of otherwise useless muscle for that.

  To be clear, being on sanitation is NOT covered in the book itself (or any other canon except that one line in the film).

  However, we can figure out that it happened well before his training ended from the exact plot of the book, as I highlighted in this answer

  (short version: the book covers the last stages of his training, his first real solder assignment on mining camp; and at the end of THAT assignment, he's straight in flight to Jakku. That means his Starkiller service well predated the end of his training).

• Finn had the markings of an awsome soldier, and was on the "watch for promotion" list by Captain Phasma, even in training. Again, that was after the "sanitation" duty (or at worst, during it).

  He was even his 4-man squad leader:

  “Let me guess,” the trooper said. “FN-2187 is in charge, right?”
  “That’s right.”

• He effed it up by showing empathy, first towards a fellow cadet, then, towards striking miner leaders on his first "real" assignment.

• As mentioned in the last point, right after their training ended, they were shipped to their first post: pacification.

  They moved from the base to a transport and from the transport to orbit, traveling with another half dozen of the trainee squads, all of them in their armor and with their rifles. The rifles were new, no longer the training version but the real thing, F-11D blaster rifles, loaded with live ammunition and fully charged for battle. Their first look at the Star Destroyer, majestic and ominous at once, was through the hull windows as it came into view—almost impossibly small at first, then growing to become almost impossibly large as their shuttle sped toward it.
  “This is for real,” Nines said, and FN-2187 thought there was awe in his voice, as if he’d never expected them to make it that far.

  ...

  FN-2187 found that he and the rest of the team had been assigned to barracks almost identical to those they’d left behind on the surface. The difference was that the ones aboard the ship were occupied by “real” stormtroopers, men who ignored them entirely as they located their bunks and stowed their gear. They’d hardly had a moment to remove their helmets and settle themselves when they heard the order coming over the ship’s PA system: all hands prepare for hyperspace.
  ... “Fresh meat,” one of the stormtroopers said. “Who’s who?”

  ...

  The deployment was to a mining colony established in an artificial asteroid field collectively known as Pressy’s Tumble. Once there’d been an ore-rich moon, but the ore itself had been buried deep, and instead of setting up operations on the surface and sinking mines, some engineer with a facility for explosives had decided the best solution was just to blow the whole thing to smithereens. Those smithereens now floated in the Outer Rim system of Pressylla, along with three inhospitable planets and a red dwarf sun that made the fragments of the lost moon glow with a hellish light.

  ...

  What FN-2187 did know was that they were there to “restore order,” according to the briefing given by Captain Phasma herself. Republic agents, she told them, had infiltrated the mining operations and were both sabotaging equipment and creating dissent among the miners. The First Order’s presence was required to put a stop to it, to get the miners back on schedule, and to prevent any further delays.

• Still he was good, so Phasma decided to let him grow into being a "real" stormtrooper, and his chance to do that was to be inclded in Kylo Ren's mission together with his group.

  Captain Phasma watched FN-2187 on the monitor in her quarters. He’d stopped firing, stopped even moving, and was just standing amid the ever-changing field of moving figures.
  She sighed. She’d had such hope for FN-2187. He had shown such remarkable promise. He had shown the capacity to be special.

  She picked up the orders on her desk and reviewed them once more. They’d already made the jump to hyperspace, and she knew it would be less than an hour before they reached their rendezvous point to take on their new passenger. Kylo Ren had already transmitted the coordinates for where they would be headed next.

  ... She switched off the monitor. He’d be part of the detail when they reached the landing point on Jakku, she decided. Perhaps when someone was shooting back at him, he would understand what it meant to be a real stormtrooper, what it meant to serve the First Order, body and soul.

• UPDATE

  • First of all, I also wrote a major long answer illuminating why Finn was top of the line cadet, and thus quite in place on Jakku

  • Second, I just realized that the whole "Sanitation" thing may not be true in the first place: " Is there any canon evidence that FN-2187 actually worked in sanitation? "

Answer 2

Finn IS a stormtrooper. In that role, his first and ultimate duty is combat (this is why he is reprimanded after the initial events on Jakku. His role in sanitation is likely his 'day job' - ie, the assignment at the base when not being a front line soldier. As to the shields, it is shown that he

did not in fact know how to lower the shields. While wishing to help, he really had the ulterior motive of rescuing Rey.

The only reason they were able to lower the shields is because the trio

captured Capt. Phasma, who actually knew how to lower the shields