The spiritual life is all about ENTRUSTING when it is HARD. If it was easy, anybody could do it. Hagar is asked to do the hardest thing: SUBMIT. She's also asked to give evidence of it by returning. Along with forgiveness, these make the top-five of spiritual difficulty.
Hagar will beautifully adorns herself in submission. I think she teaches the concept to Sarai who is later lauded for demonstrating it. In so doing these hard things, Hagar helps Sarai, and all of us who admire her.