I don't think it's really a "big thing", it's mostly for hobbyist projects (since you need to know quite a bit about software development, Linux, etc.), and for that purpose it's quite good.

It's cheap and you can easily connect leds, motors, etc. and control them via Python scripts. It's also frequently used to create home entertainment units for music, films or video games (emulators in particular), since there's an HDMI port that you can connect to your TV.