Utility of heart rate monitor for running
A heart rate monitor is mainly interesting for intense races on long distances (more than 4 kms) because it helps to manage an effort which becomes more and more difficult for the heart (cardiac drift).
Its main advantage is to verify if the effort difficulty felt corresponds to the reality. It is all the more important as the psychic state is able to twist the physical reality. Running with a low "spirit" for example, can bring an illusion of a bad physical shape.
It allows also to manage more precisely the running speed according to the relief (best control for acceleration in descent, slowing down in ascent).
However, you have to keep in mind the heart rate is only one of power parameters. Other ones, important too, are able to fluctuate with time and interact with the rate (Ref.).
The same performance can so be reach with very different rates. If you run regularly with a heart rate monitor, it is better to use the mean frequency from the first kms to define the possibility margin.
Consequently, the percentages between the minimum and maximum rate are not very helpful. Nevertheless, the beginners or the runners not reaching the performance, can use target zones (Ref.).
Finally, you have to keep in mind that the cardiac muscle is fed by diastole, at the opposite of skeletal muscles fed by systole. You must avoid to keep it too long in its maximum frequency.