When you make drawer sides you need to make a right and a left. Positioning the tails so that the slot for the drawer bottom falls in the middle of the tail makes for the best appearance. Having a 24" jig lets you set up and cut both sides of a drawer up to 12" high at the same time. Sure, each can be cut on a 12" jig, but that requires a second setup for the second side and if one side isn't cut right when you get to assembly you will have a difficult time duplicating the original setup to make a replacement side.
Doing any joints that are wider than 12", such as the corner of a toy box, will be very difficult to do on a 12" jig. You could make two 12" boxes and stack glue them together, but the dovetail pattern would likely not look right.
There are probably more instances, but these are the only two that I have run into. I'm very glad that I bought my D4R.
Central North Carolina