They effectively don’t do anything in most elevators in the US during normal operation. The ADA requires elevator doors remain open at least 3-seconds. Usually, people-moving elevators are most efficient when doors close as quickly as possible, so they start closing exactly at 3 seconds. I’ve used elevators with less common use cases — huge ones in hospitals, freight elevators, hotel service elevators — that might be configured to stay open longer than the 3 second minimum, assuming people will push the door close button as soon as they’re ready.