FM Composer is a free and open-source music software, providing a tracker-style sequencer and a completely custom FM sound engine. FM means Frequency Modulation, it's the sound generation technique FM Composer uses. Waves are modulating each other to create new sounds. It was heavily used in the 80-90's with the Yamaha DX7, theSega Genesis and many other pieces of hardware. It saw a new popularity increase nowadays with electronic music. It may be a useful tool for any composer or musician who likes experimenting with sound.
If you're interested into crafting your own sounds and composing music with them, certainly !
If you don't care about the integrated sequencer but want a sound source for your DAW, maybe.
If you are a videogame music composer, you may find a use for some projects. With the included library, developers can playback your music and alter it in real time, allowing great interactivity and immersion.
To get music made for your game, best bet is to find a composer who'd like to use FM Composer. Tell them about it. If they've already worked on video games or like to experiment with sound, chances are they will be interested. They aren't even forced to use FM Composer for the whole creation process, they can work with MIDI files then import them into FM Composer.
If you don't need custom music, the easiest way is to find some MIDIs, import and tweak them with FM Composer.