So if someone is being snotty towards someone in a way that violates board rules we can make that go away, which makes this board a better experience for everyone. (Don't we all want that?!?!) And if we think a post skirts the line but doesn't quite cross over, we do nothing. No harm, no foul, and nobody will ever know there was a notification or who it came from.
I "tattle" on people all the time.
Whether it's spam, offering stud service or puppies for sale, or people spouting off on personal attacks, I figure it won't hurt anything to click the little exclaimation point just to say "Hey, Moderator, take a look at this and see what you think." Then it's up to them to make the call.
Personally, having been a moderator before (on another board), I really appreciated it when people would alert me to things, as I could never read every single post. The only time it gets annoying is when the same person reports post after post after post simply because he/she doesn't like what someone else is saying, or disagrees with it.
There's a difference between a post that is disagreeable/controversial, and a post that contains a personal attack. Basically, the rule of thumb when posting is "attack ideas, not people".