Farmers use them to protect livestock.

Why should citizens be allowed to legally own firearms? Best reasons I can conjure up: to engage in subsistence hunting, if that's their thing. Self defense, either from other humans with bad intentions or large wild/feral animals. Target shooting as recreation. Ensuring the collectively armed citizenry can defend itself against an unlawful tyrant threatening national values and founding principles.

I would personally prefer to live in a world where everyone respects everyone else's rights to life, liberty, and property and there are no reasons at all for firearms to exist, so they don't and absolutely no one has one.

But I don't live in that world. I live in a world where people cheat, steal, maim, and kill. It would be nice to have an impenetrable forcefield protecting me from harm, but I don't have that, either. So, the next best thing, is a way to level the playing field. To ensure the smallest, least physically-imposing person can be every bit as lethal as the largest bad motherfucker intent on causing harm. I think that's the central idea as shared by The Cornered Cat.

Edit: I guess another reason would be to make an armed country harder to invade by another country. The would-be invader may think twice before laying feet upon the armed country's soil, knowing the likelihood of a strong underground and guerrilla tactics.