that's not true, there are multiple reasons one might like to, such as if you are interested in content that is blocked by one instance, particular if you want to access content on instance a and also content on instance b, and instance a blocks instance B, or vice versa.

you could want to have separate personalities, one that you discuss your expertise as a mechanic, and another that you use to talk about your expertise as a Furry.

You might want a sock account to argue with people without getting negative votes on your primary account, etc.