Why?

Early in my coaching career, I was easily frustrated by those athletes who needed to know the why of everything. When told what to do next, they would ask, "why?" It seemed, to me, like the act of a defiant athlete trying to be difficult. In fact, I once remember telling my brother something along the lines of, "if there's one thing not to do when you get to college, it's ask your coaches why you're doing something." As I've matured and (hopefully) improved, I understand that these are the athletes you want. The inquisitive ones, the ones who are trying to parse out not only what to do, but the reasoning behind it. 
To the young or uninformed coach, this likely comes across as a challenge to authority. In some cases, undoubtedly, it is. But it also gives us an opportunity to demonstrate expertise as well as set them up for success. Eventually, they will no longer be under your wing. Part of your job as coach is to teach them to fly, so that when they aren't with you, they will be OK, even if they are under the guidance of an idiot. Dan Pfaff has noted that great athletes should have a "PhD in their event." A well-informed coach, who is able to relay the appropriate information, in the appropriate way, at the appropriate time, is invaluable and will likely produce excellent results, as they better grasp the concepts and realities of training.
Understanding why something is done provides context and allows us to apply similar means and methods across different situations, changing the necessary elements based on need. Simply copying and pasting what you saw someone else do doesn't necessarily produce the desired outcome, because they may have used it for a totally different reason; which means that trying to get the result you are might not work.
Where possible, figure out the why. Then, figure out the what, the when, and the how.