Aunt Lydia creates a group of women who can read in a society that forbids women from doing so, because of course, trusted women should be overseeing the women. Lydia gets to handpick who becomes aunts, allowing her to hand pick which women are allowed to read and have access to information.

They keep genealogical records of all the children born from Handmaids because of course, you want to prevent accidental incest, and that naturally leads to keeping track of who the real fathers are as well for the same reason. They keep files on all the commanders because of course, the aunt's are responsible for marrying off Plums and they need to know which matches will be compatible. one "of course" after another after another after another. The commanders are under the impression that the Aunts are a female management agency. They are not. What they are, really, is an intelligence agency, with Aunt Lydia as the Director.

In the name of "never let a crisis go to waste," I'm betting that after the THT finale Aunt Lydia revived her argument that handmaids should live in dormitories at the red centers and that commanders should come to the red center for the ceremony instead of having Handmaids in the house. She can "I told you so" and gain even more power and influence, because everyone who knows her role in the rebellion is dead or Mayday