They can be oriented to some type of content: For example, the many feddit.something are targetting people by countries or langages (.it, .uk, etc.). slrpnk.net is solarpunk oriented, mander.xyz science oriented. Litterature.cafe is books, reading and writing oriented.
And they can offer different moderation policies: People on lemmynsfw.com probably want to see NSFW content. lemmy.world has a policy against it. lemmy.dbzer0.com allow for open discussion about piracy that many instances forbid and so on.
It you don’t see the difference in instances, it is probably that you are about fine on your local instance. But if one day, you hear about a community you can’t access, maybe that is because it is blocked by lemmy.word and you could access it from another instance