I quit using HR monitors many years ago, but then started using them again after having a poor training cycle for a marathon where I suspect I "over trained." So I use a HR monitor, to "prove" to myself that my recovery, or easy workouts -- are actually as easy as they should be.

Everybody else that uses HR monitors are mostly just a bunch of ****ers who like to think they are being scientific.

Zone HR Training is most effective for those engaged in structured training plans using "periodization."
Most of the ****ers like to think they are "doing that" too - because Pro coaches and cyclists do it.