Back in the 80's and 90's we trained without HR monitors, Power meters etc. We just had basic computers telling us distance, speed and average speed. It seemed to work just fine.
Now though, in my 50's, I use a Power meter religiously. When I got back into cycling a few years ago I used an HR monitor but now I rarely do, the power meter with data uploaded to Training Peaks for my Coach to analyse is all I need. When we are out training, it really does help to train with specific watts in mind for different sessions. I also more readily see the improvements as I get fitter, faster, stronger.
I've bought into using data to improve my performance - even Strava and segments, I find them a good tool for motivation albeit they are largely irrelevant given the variables involved and have no real measure compared to actual racing. But as an additional training tool they are ok.
I have a nice mix of social cruising, training and racing which I prefer.
