It also lets you breathe and think (and it's very comforting). Why?
Because it disengages your brain. There is very little distractions in a shower - you're forced to concentrate on a blank wall. The scrubbing of yourself is montonous and requires little brain engagement thus freeing it to think of other things - some of the best ideas come to you in the shower. A white noise effect generated by the water also isolates you from the outside environment. Last but not least, new thinking comes from a new environment (showering brings a change in location, temperature, attire (or lack thereof), and the addition of water.
So, do all your great thinking in the shower!
BTW just interested. Do you take showers first thing in the morning or at the end of the day?