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!
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)
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
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.
Is there a way to merge them all into one, so I could see posts from all of them in one place? Like, both lemmy.world and lemm.ee have WhatCouldGoWrong communities. is there a way to somehow connect them?
It's good to be prepared for your instance going down for one reason or another. If you sign up for a few that you like you can at least reserve your username for if that happens.
!nostupidquestions is a community dedicated to being helpful and answering each others' questions on various topics.
The rules for posting and commenting, besides the rules defined here for lemmy.world, are as follows:
Rule 1- All posts must be legitimate questions. All post titles must include a question.
All posts must be legitimate questions, and all post titles must include a question. Questions that are joke or trolling questions, memes, song lyrics as title, etc. are not allowed here. See Rule 6 for all exceptions.
Rule 2- Your question subject cannot be illegal or NSFW material.
Your question subject cannot be illegal or NSFW material. You will be warned first, banned second.
Rule 3- Do not seek mental, medical and professional help here.
Do not seek mental, medical and professional help here. Breaking this rule will not get you or your post removed, but it will put you at risk, and possibly in danger.
Rule 4- No self promotion or upvote-farming of any kind.
That's it.
Rule 5- No baiting or sealioning or promoting an agenda.
Questions which, instead of being of an innocuous nature, are specifically intended (based on reports and in the opinion of our crack moderation team) to bait users into ideological wars on charged political topics will be removed and the authors warned - or banned - depending on severity.
Rule 6- Regarding META posts and joke questions.
Provided it is about the community itself, you may post non-question posts using the [META] tag on your post title.
On fridays, you are allowed to post meme and troll questions, on the condition that it's in text format only, and conforms with our other rules. These posts MUST include the [NSQ Friday] tag in their title.
If you post a serious question on friday and are looking only for legitimate answers, then please include the [Serious] tag on your post. Irrelevant replies will then be removed by moderators.
Rule 7- You can't intentionally annoy, mock, or harass other members.
If you intentionally annoy, mock, harass, or discriminate against any individual member, you will be removed.
Likewise, if you are a member, sympathiser or a resemblant of a movement that is known to largely hate, mock, discriminate against, and/or want to take lives of a group of people, and you were provably vocal about your hate, then you will be banned on sight.
Rule 8- All comments should try to stay relevant to their parent content.
Rule 9- Reposts from other platforms are not allowed.
Let everyone have their own content.
Rule 10- Majority of bots aren't allowed to participate here.
Our breathtaking icon was bestowed upon us by @Cevilia!
The greatest banner of all time: by @TheOneWithTheHair!