It's mainly used for its (very) low memory footprint, compactness and the fact that it is a fairly usable mail user agent that doesn't require any GUI. This makes it ideal for simple and very low-end systems or systems with a very low bandwidth. Furthermore, it allows you to run it from an SSH-session without having to install an X server and it's highly customizable to the user's preferences.
However, mail clients like this are in decline since low-end systems today are the ultra-high-end systems of the time programs like mutt were initially created.