Well there's more to the client than just playing music. As GP said there is bloat but to me some features are very useful.
A big part of the client is for browsing through music/album/playlists/artists. Of course you could search for music outside of Spotify but it's really convenient to have it integrated in the client. There's also a social media part (definitely bloat, I have disabled it) and more hidden features like being able to control the client from other Spotify clients (e.g. on my phone) which I use all the time.
300 MB is not great but it's low enough that I don't really care.
