Yahweh promises Abram a son, but he grows old. He knows that his son will come from his own flesh (Genesis 15) but he doesn’t yet know that Sarai will be the mother (cf. Genesis 17). Sarai comes up with a plan: Hagar can serve as surrogate.
Sarai’s goal is to obtain children, but the Hebrew of Genesis 16:2 says literally “perhaps I can build [myself] from her.” For Sarai, having a son is a construction project, which builds her.
The phrasing goes back to Genesis 2, where Yahweh constructs (banah, build) Even from Adam’s rib. Sarai wants to be the new Eve, the built woman, not by being formed from her husband’s rib but by having, through her maid, her husband’s son.