In nature, a cat that is outside will go and pee in one place. The cat then buries that and moves off to another place a bit farther, and will poop and then bury that. It is an act of instinct and survival for them and ingrained in every cat. Enter domestication where people not only take cats from nature, but put them into a house and surround them with four walls. They take away the natural food of the cat, mice, other rodents, lizards, birds, and ask the cat instead to accept hard kibble with no moisture and drink water from bowls. Add to that, they put one litter pan out for the cat to use, and the cat is confused and often has accidents, for which it is punished (sometimes severely) or dragged off to the vet, or worse off cast outside because the owner can't deal.
One cat really needs two boxes, and not crammed together, but spaced far enough apart and having been put in a private area where there is quick escape. In multiple cat households there is often litterpan ambushes and having more pans will stop any type of territorial stress going on- that's why- (since it is my article)