At online check in, DL will sometimes tell the customer that the plane "may" be overbooked, and that they may need volunteers. It then asks the customer if they are willing to take compensation for VDB, and how much.
This provides DL two very key pieces of information.
1- It tells DL how many customers are flexible and willing to be VDB if the price is right.
2- it helps keep the VDB costs down, because flexible pax don't know how many other volunteers there are, and what their price is. For example, there may be a pax who would VDB for say $200, but if no one else is volunteering, they will stay quiet and see how big the offer will get to before cashing in. Doing it online ahead of time helps to prevent the pax from waiting for a higher offer at the gate, as the GA can simply call Mr. John
Doe who made the lowest offer to the desk, and ask if he is still willing to accept $200 to be VDB rather than going the auction route.