Considering that many have multiple firearms chambered for the same cartridge, e.g., 45-70, and of course you need a bullet for hunting, a bullet for plinking, and a bullet for target shooting. And, as we all know, as one bullet performs well in one gun, it might not perform well in another. That means, you have 3 guns that shoot 45-70, then you might need 3 moulds for each gun? There's 9 different moulds in .460 dia.
Then, there are other very common cartridges that the same argument works for, 30-06, 308/7.62 x 51, 38/357
Don't forget those cartridges that are shot in both rifle and pistol, such as 44-40.
"But honey, that's why despite the fact that I already have these 60 moulds, I still need this other one."