Why: It's similar to mpd, but with a lot more features and support for music streaming services.
Open Source: Yes
@DataDrake mpd is available in the software centre already despite being server software. The only difference between mpd and mopidy is the ability to stream music from net sources as well (like Spotify and Soundcloud). Given a choice in which packages I would recommend for Solus, I'd vote for mopidy.
My use for this is accessing Spotify through ncmpc/ncmpcpp, so it does serve it's purpose for single users as well.
I'd also still appreciate its inclusion in the repository. Why would Plex Media Server be included, which is PROPRIETARY server software, but not a popular open source project like mopidy? I am not complaining about Plex being included, since it's good software, but just pointing out that the argument doesn't make sense...
I would also definitely appreciate Mopidy available in the main repo. As far as I know it's currently the only MPD implementation with Spotify support, and that puts it in a unique position of being a sought after piece of software.