Paid relays can provide filtering by the nature of being paid. They can also be faster and provide other benefits.

Anyone can sign up for a paid relay. It’s totally optional and nobody is required to sign up.

The fee is collected by relay operator. As far as I know none of the paid relays are profitable so they lose money with the service provided (I could be wrong). Maybe wine relay is profitable 🤔 nostr:npub18kzz4lkdtc5n729kvfunxuz287uvu9f64ywhjz43ra482t2y5sks0mx5sz