To monitor your heart rate has multiple useful aspects that you'll be missing if you don't measure it. I could think of three, they mainly differ in the time when you use them:
- While You Are Running
To throttle your running speed
- Immediately After Running
In order get to know your own body and the signs it sends you better.
- Long Term After Running
To have a long-term log of your fitness
While You Are Running
Watch your heart rate while running to throttle your running speed so your heart rate stays in a specific range.
As a beginner in order not to overdo things, you could say e.g. "I don't want my heart rate to go above 170 bpm"
More advanced runners use specific heart rate ranges, measured as percentage of the individual maximum heart rate, e.g. 60-70% of your maximum heart rate. Different ranges train different aspects of your physical fitness.
Problems with this:
- Your heart rate has a lag of maybe 30 seconds to the actual stress.
If you are running on the streets or in the woods, the hill that pushed your rate beyond limits may already lie behind you.
This probably is not a problem when your track is even, like a tartan track or even a tread mill at the gym.
- Knowing your maximum heart rate requires a test assisted by professionals.
Rules-of-thumb ("220 minus age") or self assessments (doing intervals or run-up-that-hill-three-times) do not produce viable results.
Personally, I found this really frustrating, because I never found the right speed because of these problems, so I wouldn't recommend it to a hobby runner. I found that using my breath is better to assess my strain while running: I count the steps for breathing in and out, a quite common habit among runners.
- breathe-in 3 steps, breathe-out 3 steps equals normal strain I can endure for hours
- breathe-in 2 steps, breathe-out 2 steps equals very high strain I can endure for minutes
But please note that these steps-per-breathe values are individual and just meant as an example, your breathing cycles may be completely different.
Immediately After Running
If you have measured and logged your heart rate while running, it can help you get to know your own body better.
Usually, even as a hobby runner, you do runs of varying intensity, e.g. long slow runs, sometimes (what the heck!) blazingly fast but shorter runs etc.
When you measured your heart rate, you can check afterwards if your subjective level of stress resembles your heart rate. If it does not, then what could be the cause? High temperatures? A yet undetected infect? (Btw never run willingly with an infect, seriously!) Is it possible that you somehow did not notice signs your body sent?
All this helps you to better assess and adjust your level of stress on the next run.
I found that getting to know your own body is of tremendous importance for beginners (such as I am) : To learn to interpret the subtle signs of your body, see when you were pushing too hard, or, on the opposite, you did not challenge it enough.
Long Term After Running
If you are consistently monitoring and logging your heart rate, you have a long-term log of your fitness. It helps motivating yourself ("yeah, I ran the same distance at higher speed but with lower heart rate than 2 months ago!") or detect flaws in your training ("boy did not running on christmas kick me down, now the heart rate's through the roof after the holidays, didn't used to be like that before")
If you are healthy, I would consider it highly unlikely to actually damage your cardio vascular system when running at high / excessive strains. Long before that can happen your body will just refuse to go further. It's like trying to suffocate yourself just by holding your breath.
But what may happen if you overdo things, is that it is becoming more likely that you start having injuries due to overload - actually the cardio vascular system will adapt quite fast to your training, but the bones, senews etc. will take much longer.
I am a laymen and not a professional runner, nor do I have a medical profession.