You really don't need an account on every Lemmy server. If you just pick one that meets your needed values, you can access all the other federated Lemmy content on other instances.
The only reason I can think of to maybe do this is to have an SFW identity and an NSFW identity, and then you would just have your SFW on an instance that blocks NSFW, and your NSFW on an instance that does not.
Your identity is tied to your instance for example I am @bwok_bwok@lemmy.world and anyone who interacts with me from another instance can see that. And if someone chooses the same name at another instance they are a different identity, and the two don't really cross-pollinate, just make sure to include your instance with your username whenever you share it.
Another reason is not knowing which instance was going to catch on fire today π₯. You can only post from your own instance, so being able to switch - at least now in the early days - is a nice perk