Every email provider has different pros and cons. You might have switched from Hotmail to Gmail so that you dont have to delete emails every week because you’ve run out of space again. You might later switch to protonmail so that you could get some more privacy instead.
Likewise, you can choose an instance based on your specific needs and preferences. If you want to be in a communist echo chamber, there’s more than one instance for you. If you care about something else, there’s probably an instance for you somewhere out there. If not, you can make a special instance for all the furries who enjoy playing polo, eating banana pizza and listening to trip hop.