I seem to have trouble keeping my attention on the task at hand if there are little things flashing in the corner of the screen, or even changing color (like, "you have new email"). The less I have on the screen, the more productive I seem to be. It is for this reason that I've been using "tiling" window managers since about 2003.
I've been using dwm since a friend introduced me to it around 2008. What I like most about dwm, aside from helping me stay focused on the task at hand, is that it gets out of my way. There are no buttons to click, and no windows to drag around or resize. In fact, I hardly ever use the mouse anymore, outside of my web browser.