To add to Paul’s comment, find your resting (minimum) heart rate and your maximum heart rate (after a long sprint or end of an FTP test for example). With these values along with your age, you can find your heart rate zones. There are web sites that can help you with calculating your zones.
With your heart rate and FTP in Zwift, you can effectively get the most out of your workouts or determine what target zone you were in during a normal ride. I’d recommend having these values up to date in any cycling app you use (Zwift, Strava, Garmin, TrainingPeaks, etc.).
Personally, I’ve seen my VO2 max increase over time using these values. An oversimplification of this is view the same workout from months, or even years ago. Compare your power and your heart rate for both. If you’re training well, you most likely are able to put out more power at a lower heart rate. If you only had power, you’d only have one part of the equation = not enough data to analyze your fitness.
If your heart rate monitor is faulty, you should replace it as it adds valuable data when training with power.