A power meter will give you the ability to suffer. You still have to be willing to suffer, but it will let you know when you're not working hard enough (which for me was just about all the time.)

Power meters will also let you know if you're going easy enough on recovery days (you're probably not.)

It's great for training. Find your FTP, set up zones, print and put on stem if you want, find a training plan and ride. Review your data, track your data...

It also gives you a way to monitor yourself on hard group rides.

It's way more consistent than hart rate, as HR can change day to day, with rest or diet.

It's make me quite a bit stronger this season. I went with a Quarq Elsa, which is stupid expensive, and still believe it's worth every penny.