Self-basting is another way of saying enhanced, which means they've been brined or injected with a solution to help retain moisture. I don't think most true BBQers find it necessary for pork butts, or, if so, prefer to do it with their own seasonings rather than pay extra for pre-injected product.
Maybe in this case, they're doing it to make up for a leaner than normal butt.(?) If the price is right, it's worth a try.