Title, basically. Back on Reddit, there was a way to add a user as a friend. It made it easer to identify people (Apollo would highlight friends differently), as well as to see what they’re up to across all communities. There was even a feed for all posts by all friends, which was really useful.
Reddit didn’t always have that, and Lemmy I think is in many ways meant to resemble Reddit before the introduction of user-focused features. Kbin has the option though.