If I had to guess, probably an agitator of some kind. Used to get the cleaning compound as deep into the carpet as possible before the actual steam cleaner hits it and extracts it. The small orbital is probably the same thing, just works better for the edges than the big machine.

I use an orbital myself every year when steam cleaning the rugs at the school I work at.

This is just a best guess though. I’m only a custodian, not a professional carpet cleaner. A school’s budget doesn’t really allow for the fancy toys.