1. To look scary. Their helmets, and Darth Vader's, look like skulls for a reason, to look scary. This tells the audience that these people are the bad guys.
2. To dehumanize the people in the armor so that the designated good guys can kill them without the audience losing sympathy. If the "evil" people and the "good" in star wars weren't so clearly deliniated, the actions of the good characters could be seen by the audience as reprehensible.
Well, they are clones. It's not like you're going to run out of them.
“One armed wampa”
The comments are great...
Wampa lives matter!
“Your honor, in Luke’s defense, the wampa was going to eat him...”