Why is punctuation and grammar important?
Both are important.
Let us eat Grandma.
Let us eat, Grandma.
Can you distinguish the differences between the two sentences?
The second one is correct. Just one COMMA can make an odd like we are going to eat Grandma!
Now, consider it:
It is me.
This is a common expression by native people or colloquial style it is correct.
Who is there? Answer will be ----It is me!
But grammar is wrong.
Like after a copulative verb we need a subject pronoun---it is a rule of grammar.
So, in your exam paper if you write it is me---you will have a ZERO score.
Like these two examples I can give you hundred examples too.