none said:

(agree with LexX's the set was already built comment)

Me too. I always go with an answer based on movie production rather than 'in-universe' answers. As far as why the bounty hunters at all, I think production wise it is a matter of finding a good spot to introduce Boba Fett and other cool characters in costumes.

The Caligula comparison is interesting. This makes the most sense as far as the plot is concerned. It is easy to see Piett and the other officers have such a disdain towards the hunters, especially if Vader is trying to rub it in their faces.