Why you write comments in your source code?

Comments lead the developer to wrote bad code.

If you wrote a good source code, it will be readable without comments.
For example, if you wrote a method called:

DrawContent.AfterDivisionBy2(4);

I think you don't need to comment with a line of code like that.

So, when is useful wrote comments?

Comments are useful when you are working with a old bad code where you (or your manager) want make a lot of refactoring.

Anyway I suggest to you to review the code before comment it.