Big, Small, and Medium. One is just right(of course it depends on who is doing the choosing). A flats angler might not have a need for size 24 midge larva patterns or tail water junkie dozens of deceivers. A steelheader may need tubes and string leeches. Blue water fans and guides need big and water proof. There are lots of situations in the angling world and we have options.
Pictures can give you an idea, feel one and you'll see why some of the most experienced anglers in the world use Cliff Boxes.