Some of the basic ways it can be helpful would be calculating maximum HR during a workout, which is an indicator of aerobic capacity. HR can also help you better estimate calories expended during a workout. If you do HIIT or interval training, HR can be a benchmark for starting the next interval (ie - sprint 400, rest till HR drops to 120, repeat 4x). Also, measuring the time it takes for your HR to normalize after a workout will show recovery capacities. Measuring the time it takes for your HR to return to your average resting HR after a workout will help you see how well your exercise is in helping your achieve EPOC (excessive post exercise oxygen consumption) and recovery.

When you get more advanced, measuring HR can help you determine lactic, oxidative, and aerobic thresholds so you would be able to understand how hard you can push yourself on a single energy system.

Lots of things that HR can be used for. Depends on your exercise goals.