So your knocking something you seem to know nothing about?
Books have been written on training with power so if you're genuinely interested then you might want to get hold of one.
But to briefly answer your question - if you know you can hold 300W for an hour then your FTP is 300W. You can then use that number to set your training zones accordingly to target different systems, in the same way that people use HR and RPE, but without much of the guesswork due to e.g. cardiac drift.
There's much more to the analysis of power than just the average. Again, books have been written on this so I won't attempt to explain here.
I'm very new to training wit power, but to me at the moment it will allow me to track improvement and see the (hopefully) positive effects of my training.