When we document, we know what we have actually done. Sometimes, we are so task driven that we get carried away by our own levels of activity. We think because we are always busy, we are accomplishing what we set out to do. By documenting how we spend our time (and analyzing it too!), we separate the time-wasting from the productive. By documenting, we give ourselves the data we need to think critically about what we are doing and whether it is achieving our goals or not.
Perhaps more important, when we document we can measure change. It is tempting, and all too easy, to ascribe trends in society to our own efforts and awareness raising. Documentation protects us from that fallacy and gives us the tools to see the past and the  present somewhat objectively, making it possible for us to contribute to the future in a more concerted and systematic way.