Differences in instance-level topical focus and moderation philosophy, for one. Also physical location and “vibe”.
There are a certain set of bigoted and/or extremist instances that I would judge an instance for not defederating from. There’s a reason I’ve stuck with .world even through some turbulence. Some heinous shit shouldn’t be tolerated by anyone.
People that don’t check what community a post came from on their home feed and just upvote it if they like it.
Full disclosure: that was me just now until I opened the comments, realized, then took it back. It’s very easy to miss sometimes