After much research, I've finally found an answer to this one in the "Once Upon a Galaxy - A Journal of the the Making of Star Wars : Empire Strikes back".
As recounted by Alan Arnold, the original reason that Chewie and Leia were in the chamber was to act as leverage on Han Solo to remain compliant when testing out the Carbon freezer.
Had Fett killed Chewie, Han would likely have either tried to escape or at the very least injured himself trying to kill Fett, rendering him useless for the test. Vader prevented this, sensing that the situation could be brought under control.
(Now the scene is Kerch's [Irvin Kershner] trailer. Harrison has
arrived to talk further about the scene to be filmed this morning. It
is 11 A.M.)
IK: You see, Harrison, one thing I discovered that is going to affect us crucially is the fact that you have no way of knowing that
you are the one they are going to put in the carbon freeze. They bring
all three of you in, but you don't know anything. The princess doesn't
know anything. She just senses danger. None of you has ever been in
this place. You don't know what this place is. That's the reality of
the situation. So we have to add some lines.
HF: Yeah, there's something missing.
HF: I don't have any lines. I'm on my way to an appalling fate and, well, it's no time for a speech, but surely I should say
something, try to talk my way out.
IK: You've got heavily armed people all around you. Why do you think that Chewie and Leia have been brought in, too? I know why they
are brought in. It came to me last night. They are brought in so you
will not make problems. If you try to make a break, if you try to jump
them, try to do anything so as not to go into that pit, they'll kill
Chewie and Leia, too. They use them to subdue you.
HF: But they don't use them well enough, do they? Chewie tries to fight them off, but I don't raise a hand to help him.
IK: No, actually you stop him so he won't get himself killed.
HF: But I stop him before I know that if he fights he'll get himself killed. It's in Han's character to join Chewie in the fight.
The bargain I'm likely to keep quiet for is if I'm convinced that Leia
is safe because Lando has taken a shine to her.
IK : So we need another scene