It keeps you clean
You get nice and sweaty playing ITG, so naturally you develop for yourself a regular showering and cleaning routine. It sort of forces you into it, because there's nothing less comfortable than going to bed in your clothes and without showering first. I know for a fact that the quality of my sleep is considerably reduced if I do.
Although it's not like you need to shower multiple times per day, just popping in immediately after playing feels like being reborn. It's also super easy to clean yourself down because your skin is already loose and wet, and your pores are already open.
I really love getting into a cold shower straight after playing.
It's more than just getting clean
On top of that, I find that being clean helps me loosen up and feel more in control and focused. It's nice sitting in front of the computer in some fresh clothes after a shower. I was playing this evening and got in, and now I'm typing up this post, feeling relaxed and accomplished for pushing hard in my session.
I also found that the water helps with recovery a little, particularly cold water. I'm a proponent of the ice bath, and although I don't go to that extreme, I do like to douse my skin with cold water, particularly around the thighs and triceps where the fatigue will be greatest. By applying something cold to the muscles, I help them recover quicker for the next time I can play.
There was one thing I noticed the other day after a long session: I was still sweating after my shower. I found this a little annoying because my skin kept wetting itself whilst I was trying to dry myself, but I suppose it means I was pushing myself extra hard that day. I did play a number of unusually difficult songs and I must have had some excess energy to burn off.