Others have already mentioned key points. For me it is simply a better indicator of short term changes in your body.
Plus, most (if not all) experts asked on TR podcast have answered that HR is a superior metric of effort.
The HR monitor is like the tachometer in a car. Can you drive a car without a tachometer? Sure, but a sports car with a manual gear box is better with a tachometer.
Maybe Iâm old school, but I had an HR monitor before I ever had a power meter. I keep an eye on it like I keep an eye on the tach in my car.
I also did dfa alpha 1 testing for LT1 and I found it pretty reliable at 120-130bpm so if Iâm trying to keep things under LT1 I keep an eye on HR. If I do endurance+ (edge of zone 3) intervals during an endurance ride, I try and maintain 130bpm.
hmmmâŚinteresting. thank you everyone. there are for sure some uses I hadnât considered here.
prob will put a HR monitor on my Black Friday list.
My brother used to run ultra marathons for GB. He based his training paces, and then his racing paces, purely on current heart rate. Kept him in the GB team for ten years, so clearly worked well for him.
I think HR is more valuable than Power for training and would defo choose HR over Power if I had to.
To the unpopular opinions thread - off you pop.
For running yes heart rate and pace is all you need - main difference is running pace is a lot more useful than speed is on a bike.
Cycling power is more equivalent to running pace - which is why it is such a game changer for cycling training.
The difference to cycling is, though, that there is no running power* and hence, you have to use heart rate and pace in your training.
* Of course, I know that you can get running power displayed on your various sports/smart watches. But the numbers are not interchangeable, i. e. you cannot compare Garminâs running power figures with Appleâs. Thatâs fundamentally different from cycling where it is clear what you measure.
HR is the tach
Power is the output
both are useful â you have to look at the relationship between them, and triangulate it with how you feel
perceived effort is far more important than most of us want to think.
This x1000. Probably a thought for another thread, but I started structured training 25+ years ago in a garage with a training plan on a spreadsheet, a crappy dumb trainer, a box fan, a Timex Ironman watch to time intervals and a boombox. RPE was the name of the game for intervals. About a year or two in I added a heart rate monitor, which helped a bit to refine RPE. I am not convinced that with all the power meters, erg modes, and AI training prescriptions that my training is truly any better today than it was back then. Consistency is still key. Listen to your body. Figure out what the different zones feel like. Give it a little push on days when you can and pull back on days when you arenât feeling it.
If you stick with Strydâs data, running power is extremely useful. Iâve used a Stryd pod for training and pacing races, and it works so well.
Similar to cycling, running paces can be tricky when youâre not on a flat, smooth track, and power data can solve that issue.
It was Spinnervals for me. I was always cooked. 
Heart rate is a great metric to manage longer zone 2 efforts. Heart rate drift is real and can help aid in the base training season when compared to power.
Ha! I did a lot of Spinervals back in the day (no power, no HR). And I have to say, I was fit as hell just using RPE.