Those stages in a shoot 'em up where you are presented with lots of things to shoot that earn you loads of points, defended by only a few weak bad guys, and a pish easy boss. I suppose the idea is that this is the enemy's supply caravan, and you get to shoot the fuck out of it. (In Area 88 it really was a supply caravan.) They give you a quick respite from the heat of battle, a chance to rack up some easy points, and hopefully get your ship powered back up to a decent level. For a good example, see Super Aleste.