Audio tracks are for audio. MIDI tracks are for midi.
MIDI (Musical Instrument Digital Interface) is not music, but instead, is a language that contains instructions for a sound module to make music. For example, a midi "note" is not sound, but it will tell a sound module (synthesizer, etc.) to play that note (say, an F# half-note, on the piano, at volume "x"). MIDI tracks are useful for composition, arrangement, and editing. When coupled with an appropriate sound module or virtual instrument (VSTi), midi can produce very nice results. You can "play" in the midi notes via an external keyboard, or "draw" them in with the software. They can then be edited and assigned to any virtual instrument or synth of your choosing.