Yes, this rating is important. The higher the rating number the better that pitcher performs with men on base. A pitcher with say a GR of 5.5 and a CL rating of 7 will perform much better overall than one rated 5.5-3, for instance.

There is a brief explanation of this on the game's home page under the FAQ section. It's addressed in answer to the 2nd. question under the Questions About Playing The Game category.