Why should you remove the core of garlic? I have read it is better to remove the core of a clove of garlic. I actually see no reason for doing this. Does it have a different flavor, or is there some other good reason for it?

-edit-
In addition to the question as reaction to the answer 'because the green middle is bitter', I was wondering: 
-Then should you only remove the core when it is green, and not if the garlic is still very young?
-Is there some easy way to remove this core?
The green shoot in the middle of a clove of garlic is said to have a somewhat bitter flavor.