I think you have to understand grammar in order use your language properly. Of course, everyone can get by without that understanding, just like you can drive a car without understanding what goes on under the bonnet. But when problems arise, you really do need to have some idea about how the whole thing works.
When you consistently get it wrong, when your use of English shows you have no idea how your own language works, then you lose some degree of credibility. You might not realise it, but it could be why you didn't get that job, or why you failed to impress that person etc.
For instance, Greatnan posted:-
^I was appalled when I heard a teacher at my grand-children's prep school 'correcting' their English.
Grandson: He gave it to James and me.
Teacher: No, we say 'He gave it to James and I'.
I believe this is known as 'the knob's pronoun'. I wanted to say to her 'Would you say 'he gave it to I'? but I did not want to embarrass my grandson. Needless to say, the children were soon removed from the school.^
A school lost some pupils here. Did they know why? (I do hope you told them, Greatnan, not that it would make much difference, I suspect)
Making life to easy for our children at school by not expecting high standards is doing them a great disservice. It is unforgivable. Allowing ignorant people to teach or influence our children is appalling.
This is not being pedantic. It is wanting the best education for our children, to give them the best start in life. Why would anyone want to compromise that?