Q: If the #Fediverse allows different platforms / software / services to interoperate, then why have multiple accounts everywhere?
A: Features.
Example:
I have a #Firefish account at @youronlyone@firefish.social
I also have a #Mastodon account at @youronlyone
There is also my #Pixelfed account at @youronlyone@pixelfed.social
Let's not forget my #Bookwyrm account at @youronlyone@bookwyrm.social
While the above examples can be used for images, Pixelfed was built for, well, images. It was made to cater for uploading photos ah sharing your digital artworks.
Meanwhile, Bookwyrm is best for, can you guess it? #Books! It features bookshelves where you can categorise your collection. You can also track your reading. Leave reviews. #Authors can too, if they want.
It's no different to comparing the old #Twitter vs #Facebook vs #Instagram vs #Goodreads. All of it can host photos and albums, but Instagram is focused on sharing images. Twitter for #microblogging. Facebook for socialising (or ads). Goodreads for books and reviews.
The difference? The latter are disconnected. While the former, the fediverse, are interoperable.
Remember, you don't have to create multiple accounts. Keep one firefish or Mastodon or #Streams or #Friendica or #Hubzilla account, you can still connect to users using Pixelfed, #Rebased, #Pleroma, #PeerTube, #Misskey, and, if they will ever launch #ActivityPub support as they told us, #Tumblr and #Flickr.