Example: If you have a friend who likes Jamba Juice, and you wanted to get the person a gift card.
Are you going to spend the time to actually find a Jamba Juice and go there to buy a gift card? Or are you going to just click "buy as a gift for Friend" on Facebook?
Another option is to buy the gift card at another store, but then you're just filling Safeway's data hole or Target's data hole instead of Facebook's. Is that better?