One answer would be: “Because it pleases me.” (To use the apocryphal words of Mulay Ahmed Muhamed Raisuli the Magnificent, Lord of the Rift, Sultan to the Berbers (as played by Sean Connery in The Wind And The Lion)
I like that answer, it seems sufficient for many things.
Another answer would be:
"There's no requirement to justify why you're able to exercise constitutional rights. I and everyone else are able to go to church, they're able to vote, they're able to speak their mind. Even though the city might not like it, we have that right." (To quote Brad Krause who was recently found not guilty after being arrested for disturbing the peace by openly carrying a gun in a holster while planting a tree in his yard.)
I like that reason too, even more for some things.
But in the case at hand, the cartridge was designed for hunting African big game.
I also have the right to burn money in my bar-b-que pit, but it makes about as much sense as that rifle.