lain (nprofile…00ws)
>doesn't the fetching via followed's favorites already add enough things?
Not really. To give you an example of my usage, lets say that I don't follow that many people, because I don't like an overly crowded home tl and I have a single-user instance. In other words I prefer my home tl to be exactly what I find interesting. I also like posts from users on other instances like SPW and FSE, but not enough to follow those people, so I subscribe to the relay.
In this way when home tl gets boring or slow, I can check twkn with much more posts from instances I'm interested in and therefore posts I'm probably interested in. And I don't have to rely on someone else to interact with the post before I see it.
I don't see relays as a way to get as much posts as possible from as many people as possible, but as a way to get posts from instances where users I'm interested in are.
Its benefit kinda wears off when you establish yourself some kind of a fedi circle, but they are a great way to find people on other instances you didn't know about when starting a small instance without an established list of accounts to follow. And a way to "just fill it up with something interesting". Personally they helped me greatly to find interesting people when I returned to fedi and all I remembered was Moon, Pete of FSE and that was it.
Also I think it's a way better solution than what we had before (followbots).
>What relays are there?
Every Pleroma instance has a relay enabled by default. Mastodon side has some pretty big ones I think feld (nprofile…2wy3) follows (don't remember the domains of those).