The Why has not really be defined. I can think of three whys.
-To train
-To perform
-To see numbers.
I think the latter is what most have a PM for. You can train with cadence, HR, Power - weights and different methods in each. I think the weights and miles on the bike give the best results per time spent. If the goal is to perform, there is a lot more than power involved.
This guy below trains a lot of cyclists to get faster. His speed training is with weights, not on the bike. HE told my kid he needed about an hour a week on the bike if he wanted to do pursuit. Our '84 Olympic Gold Pursuit winner did the same (train with weights).
