Why do we need grammar Nazis

To discourage the degradation of a given language. That said, people should be polite in their corrections.

I agree with you people should be polite in their corrections.

Hopefully someone will get that:

UGG!

If you walked outside with your fly undone, or something else wrong, would you want a kind stranger to tell you?

Bad grammar and spelling is the internet equivalent of lack of attention towards your personal appearance in real life. People correct you in order to help you make a better impression on others.