It is quite surprising how many people don't have a basic grasp of grammar and punctuation. I have read emails from company directors that can't use 'there & their' properly and 'to & too' it might be trivial to some people, but it is noticible.
I've noticed it's mainly people who don't read a lot of books who can't spell/punctuate that well.

I know someone who failed to get into the police because they had a few spelling mistakes and punctuation errors in their written essay.