A BIN selection ends the listing altogether. If the buyer never pays, you have to wait....then open a case....then wait....then close the case....then relist. Immediate payment eliminates all of that. The listing stays live, until the buyer has paid. Money in your pocket, item out the door.
A OBO selection will leave the listing open until you have agreed upon the price. The buyer is not bound by the immediate payment requirement.
I guess it wouldn't hurt to add this to all my BINOBO listings...given that the BIN price is only selected about 8-10% of the time, this still can help guarantee payment for those purchases. Although I have plenty of OBO's who decide not to pay after sending in their offer that was accepted; in which this selection of "immediate payment" would not change a thing on.