I do it for the following reasons:

1. The big main lemmy servers can and do go down regularly. having accounts on other instances still gives me the ability to log in and participate on the communities I care about. I sure do wish the underlying federation logic allowed for associating selected logins on various instances together somehow.
2. Testing: often, I will create a post or comment into a community hosted on a different lemmy server and not see it update. When this happens, I'll log in on the other instance to see what that post or comment looks like from there.
3. Insurance against defederation: I participated/followed some beehaw.org communities before they defederated, and then I was forced to open an account there to continue participating in those communities.

Taking some load from the biggest servers such as LW. I still have a community on LW however, and mod with my local account

Well, I can’t answer for them but this situation in particular makes it nice to have accounts on different instances. If I can’t log on to/load my lemmy.world account then I can switch to my lemm.ee account and load content there.

I have accounts on both lemmy.world and outpost.zeuslink.net. I have had a lot of problems with seeing posts from the smaller instance, while lemmy.world has been overloaded due to the massive Reddit exodus and script kiddies.

I created the account on the smaller instance first, but I'm still having a ton of issues with federation on that one (many communities still come up blank there, while lemmy.world has enough users that I can see everything when it isn't down).