I often type up quick summaries of certain informational tidbits for the benefit of myself and others. While studying subjects on my own, I compile very compact reference notes so I can quickly refresh my memory later. When teaching students, I tend to put together mini tutorials on various subjects to bring them up to speed on certain troublesome topics more comprehensively than they may have been taught initially. I also occasionally compile quick introductions to more complicated topics for laymen who are interested in the ideas but don't have the mathematical background for the gory details.