J.W. Rinzler’s amazing Making of Empire Strikes Back book addresses this. Like you, director Irvin Kershner and Harrison Ford really felt it made no sense as written, so they came up with solutions on the fly.

If memory serves, Vader is trying to keep Solo from doing anything rash. With Leia there, Han has to comply lest she be hurt. Solo also tells Chewie to stop fighting and protect the princess on his behalf. Now Chewie too is under Vader’s control.

It’s also another way to tempt Luke. If you remember, Vader only tortured everyone on Cloud City so that Luke could sense their pain and fall into his trap. The emotional turmoil over the Carbon Freezing is yet another lure.

If you haven’t read Rinzler’s book, I HIGHLY recommend it. There’s a real-time transcript of the entire Carbon Freezing story conferences, including Carrie Fisher slapping poor Billy Dee Williams HARD during rehearsals! You really see how great Kirshner was with his actors, and why Empire works so well.

Hope this helps! =D