What Is Grammar Checker?

What is an online grammar checker?
It is very similar to Spell Check in a word typing program.
It tells you of mistakes you’ve made while writting that you are unaware to be incorrect.
For example.
How are you.
Are you able to tell whats wrong?
Well, the program would signifiy that there was incorrect punctuation, and by doing so would help to avoid doing so in the future, by correcting you.
How are you? – is how to correctly write that.
An english grammar checker would also be good for professionals. For even though they are professionals, they are still able to make mistakes. Copywriters, journalists, etc… are all able to make mistakes while writting. They may assume its right unless they double check their work, or if they had a program to point out such a mistake.
How much easier would it be when writting an email to make it look professional with an instant grammar checker?