The idea is that your mind can accept only so much ``new data'' in a single session. Playing with something doesn't introduce very much new data, but it does transform data in your head from the ``new'' category to the ``familiar'' category. Reading documentation doesn't make anything ``familiar,'' but it refills your ``new'' hopper.
Most people, if they even read documentation in the first place, never return to it. They come to a certain minimum level of proficiency and then never learn any more. But modern operating systems, languages, networks, and even applications simply cannot be learned in a single session. You have to work through the two-step learning cycle many times to master it.