I don't believe that they are acutally used in 
the animation and are often times cheaper than 
the production cels.  (But I could be wrong.)

According to animation pro Scott Frazier:

"The character designer creates a settei or 
model pack which contains each of the characters 
and defines the costumes they wear and what they 
might carry. The character designer should 
provide as complete a model as possible for the 
animators to work from or it becomes easy to go 
off model and makes the job of the animation 
supervisor (see below) even harder."