He that studies books alone, will know how things ought to be; and he that studies men will know how things are.

Theory tells you how a thing is supposed to work and people tell you how it actually does. The gap between the two is where most plans fail, because the written version leaves out incentive, fatigue, pride and the fact that nobody read the document. Neither source is sufficient alone. What you get from books by themselves is a tidy model that no situation obeys, and what you get from watching people is instinct with no structure under it.

Whatever you are studying at the moment, add the other half. If you have read about it, go and ask someone who does it daily what really happens.