Once we got the power meter, we did a field test to find an FTP of 280 watts, and then established his power zones. I prescribed some Zone 2 riding to help build his aerobic endurance, (goal was at least 50% of ride time in Z2 on a flat to rolling course, 151-212 watts), since he has a VERY long road race in a just a few weeks.
When I went to the review the file, it was clear the goal of the workout was not accomplished. You can see what I noticed here: (click on images to enlarge)
This post is not meant to throw the athlete, "under the bus", but help to show the power of data as a communication tool between athletes and coaches. This athlete is learning how to train, and power data is helping to facilitate the learning.
The data is helping me to assess his strengths and weaknesses, and make sure the training addresses those, allowing for precise monitoring and prescribing of training for the athlete.
If you're not using data, this is just some of what you're missing, and how you might not be training as precisely as you think.