How much and Where is the punctuation important in English?

I wonder what is the importance of the puctuation in English grammar writing.

FOR EXAMPLE :  In non-defining relative clauses , A comma is put in front of which clause.

There has been a decrease in deaths from motor vehicle crashes over recent years , which is partly due to improved safety features in cars.

Could you explain the other subjects that has punctuation such as comma  dot?

Mister Micawber wrote:

The comma is correct in your example sentence; non-restrictive clauses are preceded by a comma.  The rest of your question is far too broad to deal with here, and I can only refer you to the many grammar sites on the internet-- for instance, THIS ONE.

thanks sir.