True it's easy to overthink this stuff, but it's also true that a lot of the huge increases in the ability of amateur cyclists and triathletes to approach professional caliber times has been attributed to powermeter usage. This has really shown up in the Kona Ironman, where the gap between pros and amateurs, particularly on the bike leg, has markedly narrowed as of late, and a lot of coaches attribute this to the number of amateurs who now have access to powermeters and coaching with PMs.

Of course, powereters still isn't essential for performance. Still, having a functional knowledge of HR levels and efforts is very helpful for training. A lot of people hate the bike trainer because they have no idea how it's affecting their training or how hard they should be working on it (other than ow...it hurts.) Once you start putting power targets as an objective measure on that trainer, it totally changes how you approach it, from "ummmm, just ride until it hurts and hope you get better?" to "I know that if I'm riding at this wattage for this long, I'm definitely improving!"