Because sometimes you can be tired and you want to go hard, but you will never go hard enough. When you feel too good and want to go slow you won't go slow enough.
Taking your resting HR in the morning will tell you whether you have an infection creeping up on you or if you're a little tired. Without a HRM you will find your easy workouts getting too hard, your hard workouts getting too easy. Sometimes it's a matter of 3-4 BPM. Don't get stuck in no man's land.