A good Heart Rate monitor can do three things for you.
1. Help you effectively increase your top speed.
2. Help you effectively increase your endurance, go further if you will.
3. Tell you when it it time for more fuel.
Forget the technical terms, just know that there is a work-level point where going faster only works on your speed and eats up your endurance. For most people it takes time and effort to guess where this point is. Working with a HRM can help find this point quickly.
You know there is a speed/effort level where you can "ride all day" right? You also know that above that effort you can't keep going. Do you know where that point is? Can you tell precisely?
That is where a HRM helps the most. By working with different rides (endurance efforts and speed efforts) you slowly learn where that heart rate point is.
It takes a bit of effort and it is kind of complicated though. However, some of the newer HRM's have more features (like cadence, graphs, resting HR tests, 'Own Zone', etc) that decrease the effort and complexity.
Here is one example of how my HRM helped me learn to ride faster with less effort. (Using the same trail system over several months)
First I went for rides full speed ahead, no holds barred, and tracked my average speed and max/average heart rate.
Back then it was something like 11.4 mph av 166 max / 145 av beats per minute. Pretty high hear rates for my age.
Next I took a "base ride", by setting a HR zone with 75% of my max HR (175) as the max heart rate. The rules were simple: ride as fast as you can without going above 135 bpm and if the over max alarm goes off on the HRM slow waaay down until it dips below alarm level.
I was appalled after my first ride: 9.1 average speed, 136 average HR. 142 max. (remember, my HR alarm was set to go off at 136 beats per minute bpm or above) Yes, my average was still higher than my max limit! However, after a couple more rides I quickly learned what effort it took to ride over trail areas without greatly increasing my HR. In fact, with road riding included, I soon learned how to raise or lower my HR by 1 or 2 beats.
After 3 or 4 training rides like this I was up to 11.0 average speed, 136 max HR and 133 average.
In other words, I found my 'ride for a long time' point AND learned how go faster with less effort.
Alas, as human beings change all the time, it takes constant effort to learn where that point is today. Rarely, over time, does that point stay exactly the same.