@homegrown Maybe it's both. Content discovery on small and single user servers wouldn't work without a relay, so it's very useful. But it's my understanding that it pulls posts from servers using an interface intended for apps and users, not for relays, so it may relay posts from servers whose admins didn't intend to push to a relay. But I'm fuzzy on the details, so I may be wrong.
To help with content discovery there are other types of relays and there are also services like newsmast that are slightly more curated.