It depends on the payment model being used by the relay. Not Facebook groups as the relay... Well it just relays. Think of it now like Twitter with redundancy. Each relay is like a Twitter server. By having several relays on your list, you are less likely to be censored, and the load is distributed.

Some relays may charge a monthly fee, others a one time fee. Relay operators can and should ban npubs that spam or post illegal content. The number of relays avoid or turning into being banned for wrong think though, as it's less likely you'll get banned from them all by bad actors.

It's a system that keeps you from being subject to the whims of one person.