Lord Najib

3 things come to mind.

1) As an Enchanter of 3 years, I generally know when my mez is about to break just because I have used the spell so much that, you get the feeling "I should remez". With a timer, I could wait until the last possible second (counting for casting time) to remez.

2) When you are camping a spot and you either know or learn when the first mob in your "cycle" spawns you can then camp it. That way, once you've cleared your cycle, someone knows that you have 3 minutes left, which might be a quick afk or even an extra pull from outside your cycle.

3) Timing certain buffs to be more efficient. I have my Epic, so normally I'll use my Epic Haste and just rehaste in between pulls after a while, since it doesn't cost mana. Occasionally, VoG is better than Epic Haste and with my AA abilities I know it lasts 48.3 minutes. If I'm at full mana and 40 minutes have passed since I casted VoG on the first tank, I'll probably go ahead and start recasting VoG in between pulls. If its only 30 mins I might wait a little longer.

Now all of this can be done with a stopwatch, some form of clock. I used to use "chess" clocks to time buffs/spawns. That requires you to look away from your computer. Even 2 seconds is a long time when 3 mobs just entered camp (2 of which need to be mezzed, 1 which can flurry) all beating on the warrior.