awww what a cute thing for her to do. Don't know why she would do it, perhaps she simply likes how it feels or the noise it makes.
Gracie, when she simply can't wait another minute for me to wake up, will climb up on me and pat my cheek with her paw. Very softly, and with her whole paw, she'll just pat, pat, pat on my cheek as if to say, "Momma, are you awake? Momma, it's time to get up." Her pads are sooooo soft, it really is the nicest way to be woke up.
And all 3 love to rub then faces on us. I think it's a type of greeting and sign of "we are a pack" that dogs do.