They're more slots for attaching things (belt clips, firesteel holders, ect) without worrying as much about hole spacing. Unless you use a really thin belt or really wide slots, they're not much use for that. Even then they're not ideal. Curving the material like some do on pistol holsters works OK I suppose.
Like Chris said, you can mark a line, drill some holes, and then connect them. Those in the pics were probably done on a mill or such.