1) On recovery rides I make sure I keep it in a certain range

2) On long mountain climbs I use it to help dial in my wattage. If my HR hits 175 I'm in the red and might have to stop, so I'd dial wattage back a little and try to get my HR back to 172 or so. Think of it almost as a temperature gauge on a car.

3) After the ride, I may periodically check my historical average HR vs Wattage and look for trends over months of data. Generally it seems wattage will go up at the same HR over time. Fun way to help quantify gains in fitness.

Other than that, I ignore it. I do not use it for training outside of those 3 scenarios.