I don't learn anything new from examining a design for the 23rd time. I've checked for mistakes, but it's all based on what I already know and a few educated guesses. The only way I can learn more is by building....
Say you want to be a veterinarian. You've got to get to know a lot of animals in school, both inside and out. That requires a lot of time in the library, but eventually you have to get your hands dirty and actually touch some...