OK, I just bought a new watch, and I spent the extra $25 or so to get the Heart Rate Monitor piece. It's a Timex...you wear a chest strap that transmits your heart rate to the watch.

Question is...now that I have this thing, what exactly am I supposed to be looking for?

When I'm doing an interval workout, I peak out at about 172 bpm. That's about 92% of my theoretical max heart rate (220 - my age, which is 34). I know people tend to use HRM's the most when they are doing speedwork, but I'm not sure exactly how or why. Ideas?