Why did Lavan give Shifchos to his daughters?
Tosfos ha'Shalem (1): The Shifchos would do what a wife must do for her husband, so Leah and Rachel would be free.
Why does it say "Lah l'Shifchah"?
Ohr ha'Chayim #1: A Midrash teaches that she already was Rachel's Shifchah. Refer to 29:24:152:2 and the note there.
Ohr ha'Chayim #2: Lavan stipulated that Yaakov has no rights in her. Whatever we can expound to the detriment of Lavan, we expound.