10. Because C# isn’t your first language.
9. Because your mate on the desk next to you isn’t going to be there for ever.
8. Because activity is obvious, purpose is not.
7. Because you need to take a break from talking jargon all the time.
6. Because it may not reduce the number of stupid phone calls you get, but at least it’ll shorten them when you tell them the URL where you’ve already answered the question.
5. Because it keeps you honest: if it’s hard to explain, it’s probably wrong.
4. Because you’re not going to remember how it works in five years time.
3. Because it makes it easier to spot holes in your use cases.
2. Because the internet isn’t full of source code reading savants.
1. Because one day, someone might thank you for it.
And yes, for the record, I have been writing documentation recently.