This is basic from school.
Why not?
Two reasons: First, you are telling an attacker, "Look, my code works like that, and this function does that, etc., etc." Without comments, let the attacker read and try to understand everything alone, if he can.
Secondly, it's easy to copy totally or partially, you spent hours coding and anyone can say "Oh look a nice function; it's what I was looking for."
The normal behavior should be keep comments for yourself in a development code copy delete all comments for production.