Great Info thanks for sharingOff the top of my head:
1. If you're just starting out, you'll probably find that you play more evenly, as soon as you've developed the necessary coordination. The extra limb acts as a sort of personal metronome, because you're that much more aware of the 8th or quarter note pulse throughout a song.
2. In rock or pop, you can keep that left foot hi-hat going as you play a fill, which adds fluidity to the overall sound. This way, there isn't a "gap" whenever you depart from the beat to play an accent with the band, or a fill.
3. If your soloing or fills get too "out there", the rest of the band has something to listen to, in order to stay on the beat. It's a nice skill to have when you're playing with people you haven't played with before, or that aren't very advanced.
4. Sometimes, the song just sounds better. A steady left foot hi-hat may even be on the original recording.