It's another useful tool, particularly if you train alone and want to stick to an effort based plan, you prefer unmeasured trails over practicing on a track, you are away from home at a place where you don't know any distances, or don't have access to a track.
Depending on your situation, it allows you to be more flexible than a basic stopwatch.
It keeps you honest about keeping "easy paces" easy.
It enables you to do "interval training" on any unmeasured trail, based on time and effort, rather than time and distance.
If you are good at "training by feel", and have measured places to do "time and distance" based intervals, a HR based watch does not train you better than a watch with a timer.
GPS watches are useful if you don't know distances, but want to (providing you get a good signal -- an issue in cities and woods), and if you want to download a course onto Google Maps.
I used a foot pod for a while, until one time when I just no longer replace the batteries. The pace and distance feedback was interesting at first, but then the constant tuning required to maintain high footpod accuracy became a burden. It enables distance based intervals on unmeasured courses.
I did see a trend in HR-fitness over the years, but once you are fit, this data becomes less interesting, primarily because you already know you are fit.