To quickly load and unload larger planes, the baggage is loaded onto these containers, called unit load devices, as passengers check in, then the containers are loaded onto the plane when it arrives at the ramp. Reverse the procedure at the destination, and now instead of hand-loading hundreds of bags and air freight parcels, you’re just loading 20-40 of these containers. If a bunch of passengers on a flight are all connecting to same flight at the next airport, all their bags can get loaded into the same container, and they just transfer the whole thing onto the next plane without having to open and resort it.