The benefit is being able to read more notes from people who you don’t currently share relays with. It’s a way to expand your social graph. Depending on your client you can usually see which relays another user is on and many have an option to add them to your account.

Some require an admission or subscription fee which help keeps them economically sustainable, and some also have different member-only features.

You can also find relays to join at https://relay.exchange