I don't compete, have no intention of competing, but I do use a Power Tap Pro+ power meter.

The biggest use, for me, is that it gives a much more accurate picture of calories burned than anything else I've tried.

I also find it to be an effective training tool. I don't do a whole lot of data analysis but I do know my FTP and that alone is a big help. I used to have a tendency to leave my driveway at top-speed, then burn out after a couple of miles of over-the-top effort. With the power meter, I now get immediate feedback on just how much energy I'm expending. That's a big help when tackling a long route or a new climb. I'm not a huge fan of intervals, but I do think the power meter makes them more effective. Again, it's the immediate feedback versus the lag you get with a heart-rate monitor.

Plus, it's just fun to play around with. You can answer questions like: does that new racing tire require less power than your previous tire? Do clipless pedals allow you to generate more power than platforms? How much power do you save when drafting someone? And so on.