It's also another revenue source.

Old system: Haagen-Dazs pays the store a large slotting fee to be on the shelf of the freezer that's at about chest height. Ben & Jerry's pays slightly less to be one shelf higher or lower. Store brands and bulk ice cream go on the lowest shelves or above head height.

New system: Haagen-Dazs and Ben & Jerry's buy ads to be shown on the freezer door. They aren't like TV ads, they're just static displays of the containers of ice cream they want to market, but now instead of poorly lit, possibly ice-encrusted containers of whatever ice cream is currently available, they get to promote whatever ice cream they want you to buy. They probably still also have to pay for slotting fees, though these might be lower if the door is a huge screen.