If you like the instance you’re signed up with and agree with their rules/moderation, then as long as your instance and the instances you want to see are federated you really have no reason for signing up to other instances.

Everyone else answered the question, but welcome!

It’s for ones that lock themselves off from other instances (look at how beehaw is, for example). Not every community is meant for everyone online. Communities can be obscure, and gatekeep people, if they really want; that’s the point of Lemmy, in my opinion. You do what you wish with it

you can comment on the external instance if your own and theirs like each other. for multiple reasons instance owners can decide to cut communications to outside ones (called defederation). recent examples of cutting communications are beehaw defederating lemmy.world (with the reason of their instance owners wanting to protect themselves from bot invasions) and lemmy.world defederating exploding heads (reason being them being a right wing cesspool)

I made an account on another couple just for the heck of it. Some have different interfaces, or are federated with other servers that this one might not be. Also it’s like a backup - this one could disappear, be down for a couple of days, or decide that only AI photos of lemmings are allowed. Who knows. But yes, it’s not really necessary due to the way Lemmy works.

If you go to the homepage and select All, and perhaps Hot or New, then look at the top line of the posts that show up, you can see the different servers they were posted on originally. Theoretically voting and commenting on those works the same as communities formed on this server. You can see a feed of only communities on here by selecting Local.

One thing I noticed is subscribing to communities on other instances doesn’t seem to work - days later it still says “Pending” and I don’t know if that’s a policy or a technical problem.

I’m signed up on four instances, lemm.ee, lemmy.one, lemmy.dbzer0.com, and lemmyonline.com. Right now I’m mainly using lemm.ee and lemmyonline.com as those perform the best for me. The reason I have four is from looking for the best performance and admin. Performance being in terms of the best ping, least hops, and best server response. Then there’s the instance settings and admin quality.

Generally it’s good to have two login instances. That way if one goes offline you have an alternate.