Answer:
Having good grammar is being able to use words well. Being able to be understood. When you have bad grammar it is hard to understand, and/or put the sentences together.
Good grammar: When I walked to the store my ice cream fell onto the ground.
Bad grammar: When I go to the store my ice cream fell.
So as you can see the first sentence is much easier to read then the second. So yes, good grammar is very important.
