Most people don’t know this, but Spotify uses peer-to-peer networking for its streaming. In other words, when you choose to stream a specific song or album from Spotify, you may get your stream from Spotify itself, or you may get it from other users.
The problem with this is that peer-to-peer networking may affect your internet plan. While many users have unlimited downloads and uploads, this isn’t the case for everyone. If you use Spotify’s desktop client, you’ll not only be downloading music, but you’ll also be uploading, if the music you’ve listened to is popular enough.
Spotify doesn’t seem to have a page explaining this, but here’s a forum thread where a moderator discusses the use of peer-to-peer. Note that mobile apps and the web player don’t use peer-to-peer networking, so if you are on a restricted data plan for your computer, you might want to use either of those.
Also, if you reduce the size of the cache Spotify stores on your computer (Spotify > Preferences), there will be less music available to stream.
So be aware that, if you leave Spotify running, you may be using up a lot of your quota, uploading music to others. Reduce the cache, and quit Spotify when you’re not using it. Or, just use the web player.