There is no compelling reason to use it, or for it to even exist. It offers no tangible benefits that we can't get elsewhere. What it does offer is pretty lousy.
The only two arguments we see used to support its existence are both badly flawed:
1) Devices running it are "affordable". Maybe this is true in some absolute sense, but relative to other mobile OSes and devices this "affordability" comes at a very steep cost in terms of usability and practicality.
Used Android and iOS devices can also be acquired quite cheaply these days (even in poorer nations), but unlike Firefox OS and its devices these used devices are comparatively powerful and much more practical to use.
2) It's "open". This argument is academic at best. We can already use HTML5, CSS, JavaScript and other so-called "open" technologies on Android, iOS, BlackBerry OS, and many other of the less-common mobile OSes. Android and some of the other mobile OSes are already open source software, so it's not like Firefox OS has any advantage in that sense, as well.
The whole reliance on JavaScript makes Firefox OS even less-open, in my opinion. When it comes to using Android or iOS, for instance, at least we have the option of using mature, non-JavaScript languages. No, I don't really consider CoffeeScript, TypeScript or Dart to be anything but JavaScript with a prettier syntax and/or slightly better type checking.
The effort put into Firefox OS would have been much better spent fixing desktop Firefox, or Firefox running on existing mobile devices. Even putting it toward Thunderbird would've been useful. But it has instead been wasted on an eighth-in-line mobile OS with limited capabilities and a severe lack of usability compared to its competitors.