Not just knowing your code works, being able to capture what the code is supposed to do for the next person that comes along. For example, this week I've been

Not just knowing your code works, being able to capture what the code is supposed to do for the next person that comes along. For example, this week I've been working on refactoring some code that had some performance problems. Digging into the code I found a lot of confusing logic and had to waste a lot of time stepping through the code trying to understand what it was doing. If there were unit tests, I could have treated it as a replaceable black box.