In the beginning there was Pleroma, the front end. It was intended to be a more modern redevelopment of a GNU Social front end similar to Qvitter (I think the name Qvitter originates from "quitting Twitter"). Then some time later a backend was developed to be a fully alternative but compatible federated social network server.
So why might you want to run Pleroma rather than GNU Social?
GNU Social is the canonical OStatus implementation, with a PHP codebase originating from StatusNet which began in 2008. It's a fine system and I've been running it for a couple of years, but the codebase does contain rather a lot of cruft. I upstreamed some patches which removed a lot of Google stuff which was either bad or no longer relevant (like the long abandoned Google Buzz). PostActiv, which is a fork of GNU Social, has also been doing a good job of improving the original codebase, making it more performant and improving the code quality. But despite the improvements GNU Social is still quite heavyweight, and especially on minimalist ARM hardware runs slowly.
State of the Federation
The fediverse has been doing quite well in 2017. The Mastodon craze earlier in the year brought in many new users, and a fair proportion of them stuck around. Often the comments indicate that new users find the federated networks to be friendlier and more helpful than on Twitter. My favourite analogy is that whereas Twitter is like going to a concert, with celebrities and adoring fans, the fediverse is more like chatting in a pub where the egos are not so inflated. The structure of a federated system makes this type of social interaction more likely. That is, the medium shapes the message.
Running Pleroma, GNU Social or PostActiv on an ARM server makes it straightforward to own your little corner of the larger social network, where you have control over what you get to see and who you want to interact with (or not, as the case may be). As I like to say, the best way to improve the internet is to become the internet.