Well what do YOU use them for?
I'm not saying there isn't a need. I just don't know what it is.
I myself use zero-clearance plates on the table saw for Melamine to reduce chipping. Although I have found the right and sharp blade is more important.
I made my router table fence 8 1/4" tall because I do route things vertically. To help balance the tall and narrow wood, precision zero insert rings are also essential. The zero insert rings also help minimize dust and chips getting into the motor of the router though the router fan cavities. Remember, routers were/are never designed to be used upside down.
For usual edging or dado work when the work pieces lie flat, zero-clearance fence is more important than zero-clearance insert rings for reducing chipping. So the zero insert rings only help minimize dust and chips getting into the motor of the router though the router fan cavities in those scenarios
The zero-clearing plate on the table saw is for work quality (reduce chipping) and also for safety when ripping thin strips (BUT wide side should always at the fence though). Thin strips can fall in the gaps and get thrown out by the blade or gets jammed and cause kick-backs or accidents.