What is the purpose of these removable "collars" on ceramic light fixtures
It looks like the metal screw socket would be touchable without the plastic ring in place. When the bulb is lit, I believe that would be hot (energized).

Since they may use the same base for different size screw fittings, the ring is possibly designed to protect from shock hazard but being a separate component allows the base to accommodate various sized sockets.

The specific scenario I can think of when this would be important is when screwing in a new bulb to replace a dead one. The circuit could unknowingly be on (since the bulb is dead), and you could get a shock when the bulb made the circuit.