Manual tranfer switches are still available. The fancier automatic switches with a separate breaker panel are used to isolate the critical circuits (freezer,well pump, gas furnace, etc.) from the non-critical circuits (dishwasher, clothes dryer, electric baseboards, etc.). That way you do not have to go around and shut off everything before you crank up your generator. Nor can the wife or kids turn on something that will overload your generator.
Since the transfer switches with the circuit boxes handle less total current than the "master switch" type, they can actually be cost competitive with a 200A double throw master switch.