Well then, there are several possible answers. Pick the one you like best.

1. Because we can.
2. Because we think we should.
3. Because we seek cartridge rifle performance from a muzzle loader, or as close to it as we can get.
4. Because slick advertisements have convinced us we need to.
5. Because it's fun to be able to say "my gun shoots faster than your gun".
6. Because we like getting beat up by recoil.
7. Because whitetail deer are darn near impossible to kill without a zillion pounds of energy.
8. Because that particular load happens to be unbelievably accurate.
9. Because our friends shoot that load.
10. Because we want to.