You will be using your code in 6 months

Code that you wrote 6 months ago is often indistinguishable from code that someone else has written. You will look upon a file with a fond sense of remembrance. Then a sneaking feeling of foreboding, knowing that someone less experienced, less wise, had written it.

As you go through this selfless act of untangling things that were obvious or clever months ago, you will start to empathize with your users. If only I had written down why I had done this. Life would be so much simpler. Documentation allows you to transfer the why behind code. Much in the same way code comments explain the why, and not the how, documentation serves the same purpose.