If you've been a developer for any length of time, I'm sure you will have encountered a situation where you've read some code, thought to yourself "wow, that's terrible, I'm going to change that". Then, several hours later, realised why it was done the way it was and actually, you can't really improve it after all. That is why we comment; to save the next person who enters the cave (and, if you're the kind of dev who spends several hours trying to understand something and doesn't retrospectively comment what you've found, then I'm afraid we can never be friends).

And please, don't assume the next person has the same knowledge of the system that you do.