To test lighting, primarily. And exposure. Think of it as an analog version of an LCD on a digital (arghhh!! sorry but get over it) camera.
Especially using strobes, the Polaroid provides a check before you burn a roll of film and wait for the processing, etc. Also gives immediate feedback for approval by art directors so they can now leave the studio to get lunch and get out of your hair.
When a studio shoot involves stylists, makeup artists, and sundry other people, yes, it is worth it. Beats reshooting a $5,000 setup.