This is more than a theory- it's a practice that's tried, tested, and works. Ask any professional athlete. Ask anyone who's used this to monitor their progress- it need not be a professional. Just anyone who's done it. It works.
You can go by feel, but feel is subjective and does very little to confirm how much progress you made. So if you want to just go by feel, do it. But if you want that edge, you'll go for the technology. My interpretation of technology is that there will never be enough of it to help you out, and the more you use, the better advantage you'll have over the novice who chooses to use less or go by "feel" when training.
Can you learn how to get to the state of someone like... say... Lance? No. Lance got so far on his own, but recognized quickly that technology was necessary to track progress. I think it was New Leaf who first began using their technology with Lance to track his training progress at age 19. Now, he's a technology junkie (see the Lance Chronicles for further elaboration), and he's better for it, and definitely, he has a better handle on his training and progression (I only use him as an example since everyone knows him).
If you're serious about training, you know what you need to do.
Koffee