Why wouldn't he just plug that one out into the PA and feed the sound to the speakers

Because depending on what you play on drum (in this case metal) you want to hear yourself better (to see if you're playing tight).
You COULD do this too (only using the PA sound) but when you hit hard, you don't really hear yourself, depend on drummers.
The drummer in my band use this combo to be sure he plays well (headphone with trigger sound + PA output for jam purpose), especially at 230-240+ bpm.
Alot of live drummer use metronome+trigger in their headphone.

Aah, nvm <.<

This post was edited by OpO on May 5 2011 08:10am