What is considered a good Essay score? How does the essay help the overall application?

My D didn’t do so great on the SAT this past go around. However, she did reasonably well (we think) on the essay portion.
She received a 7/6/6.

Would someone be kind enough to explain just what the essay adds to a student’s overall application? What do colleges use the essay score for? Is it used just for the student’s placement once admitted? Or does it actually impact admission?