There are a number of running training schedules and approaches that use fairly exact HR thresholds for various runs. The more serious you are with your training, the more likely these gadgets help. for example, one popular use is as a better gauge of recovery between intervals. instead of the old, walk half the distance or walk the same amt of time, using a HR monitor can tell you when recovered more accurately.
here is a nice article from pete pfitzinger (famous as 2 time US marathon olympian and author of eternally popular Advanced Marathoning - and thus someone with more credentials than itzly ) http://www.runnersworld.com/race-training/heart-matter
i for one love my garmin. the combination of GPS, timer/watch and HR monitor is great.
for everyone knocking these devices, i would love to see your event placings. be it running, biking, whatever you do. how did you do in your last race? i'll put my number against yours anyday.