The fins (usually called 'horns' if I remember correctly) are for the piston rods that push up or down the mobile lounge. If you look carefully, you'll notice that there are 2 varieties of mobile lounges, one with horns and one without horns. And if you look even more carefully, you'll see that the ones without horns can't move up or down. They're used only for shuttling between terminals. The ones with horns are used for shuttling to/from airplanes (i.e. International arrivals and some planes that park remotely) as well as shuttling between terminals (although not too often, I think).