One of the bigger questions asked by athletes is, "Do I need a heartrate monitor?" Certainly, a heartrate monitor is a great tool for increasing the quality of your training sessions, but heartrate alone is nearly worthless.
To give a better example, let's consider building a house, and the only tool we use is a drill. You certainly could build a house, but chances are it wouldn't be a high-quality home without also using a saw, hammer, level, etc. A heartrate monitor is a single tool, in what should be a tool chest of training tools.
Many athletes judge the quality of a training session based on the activity of the heart, specifically the heartrate they reach. A common saying is, "I wasn't able to get my heartrate up," or "I got my heartrate up and it stayed there!"
The problem with these statements are that heartrate is extremely variable, based on many, many factors outside of intensity. Whether heartrate is able to rise might mean great intensity, or it might show a lack of fitness. If heartrate doesn't rise, it might mean you're tired, or that the body is fit, and able to effectively handle the load you're giving it.
If we take an unfit person, and put them under physical stress, their heartrate will rise exponentially. This doesn't necessarily show a quality session, only that their body is not fit. Again, this illustrates how heartrate alone does not give you much useful information.
So why use heartrate at all? Remember, heartrate monitors are a single tool in a toolbox of training tools. Heartrate is a great indicator if it is compared to something objective, such as watts or pace. If your heartrate goes down at the same watts you rode 2 weeks earlier, then you see fitness! If we judged on heartrate alone, we would be wondering if we were any fitter than 2 weeks prior.
Inversely, if we rode at the same watts as 2 weeks prior, and our heartrate was higher, it would show itself as fatigue, or possible sickness. If we judged the ride strictly as being able to raise our heartrate, we would be hard pressed to understand that possibility without a power meter.
But what if heartrate raises and watts go up? Well, then we can take a ratio of average heartrate to average watts, to see how the workout went. (We can generally replace watts with pace for all these examples as well, whether running or swimming.)
What about heartrate compared to perceived exertion? Good question! Remember, heartrate needs to be compared to objective measurements, and perceived exertion, by definition, is subjective. You may feel great, but that is not an accurate picture of what is going on.
The more information you collect, and the more variables you are aware of, the better picture of fitness and quality training sessions you will have. Certainly, knowing your heartrate is a great thing, but heartrate alone is not enough information without something objective to compare it to.