Pros once used Polaroid backs to preview studio strobe setups. Now you can do this more easily and cheaply with a digital camera. Even when shooting film, you can preview a complicated flash setup with digital.
I usually use hot lights in the studio, so I don't need to preview. I use a Polaroid back (with Fuji instant) in my Bronica to give portrait customers something to take home from the session, just a treat for them to enjoy.
Like Halford, I love playing with Instant, but the Polaroid back in a 6x6 camera only gives a 6x6 image. For street work I shoot either a modified Polaroid Automatic 100 (with the original lens and shutter replaced by a Kaltar) or a Mamiya Universal, both of which give a full frame image.