[MartinBrody]

This is from Lee's website Interchangeable dies

Frequently, dies from caliber may be used to load other calibers. Due to the locations of crimping shoulders, the operation may only work for dies of the shorter caliber to the longer.

This addresses the whole issue of using a special die set to load a magnum cartridge. It seems the reverse isn't true, you can't use a magnum die set to load a special cartridge.

Here is my question: Why make the magnum die set? If the special set loads both, is there any advantage to buying the magnum only set?

[Car Knocker]

Magnum die sets are made because people buy them. If there was no demand, there wouldn't be any incentive to make them.

[ClarkEMyers]

Some people like the alignment with the die against the shell holder.