Doesn't happen very often and she will be a heavy favorite against 20th seed Muguruza in the Final on Saturday, though Muguruza did beat her in last years French Open.
This would be the second time she has been the defending champion in all four majors if she pulls it out. She has not yet had a calendar year Grand Slam, but would be the heavy favorite at the US Open.
Also of note, if she wins she will have 21 Major Titles, which is just 1 behind Graff in the open era and 3 behind Court all time.
Interesting to see her place in history when all is said done though Graff's 8 titles in 9 majors is something I don't think we see ever again. Just absolutely amazing run for Graff from the 88 Australian to the 90 Australian.