What use does a Heart Rate Monitor have if you have a Powermeter?
You can tell when you are knackered.

Power indicates your actual output.  With a baseline of previous power and HR data, you can establish a model for what you should be able to do.  When your numbers start to fall way outside your model/baseline (you normally produce 250W at 80% of your max heartrate, but now you are only producing 200W at 85% or 90% of your max heart rate) it might be time to do something about it (check hydration, calories or just end training for the day).  Additionally in off training times your resting heart rate can be looked at as an indicator for how well recovered you are.  

Power output is a finite number that is extremely useful because comparisons can be made directly across riders.  However, your own stats bio metrics are more useful to you as an individual because they can help guide your training and prevent you from overtraining, help show your individual improvement, and let you know when you are recovered and can accept an increased training load.