Why is it important to use correct grammar ?

Why is it important to use correct grammar ? 

From time to time I find myself getting involved in a discussion about grammar. It's always the same: on one hand we have the people who argue that good grammar is important, and on the other hand we have those who say it isn't. Here I present my reasons for good grammar being important.

It boils down to this:

The purpose of good grammar is to ensure that what you write is correctly comprehended and is easy and enjoyable to read.