The answer is both. In reality, there are pros and cons to both types of boxes. Cats like deep litter boxes because they can bury their waste. The deeper the litter box is, the more likely it is that your cat will bury her waste.  It’s instinctual for cats to bury their feces. It helps keep them safe from predators and other animals that could harm them. The main advantage of a deep litter box is that it gives your cat more room to move around in it comfortably. Deep boxes also tend to be better at containing odors and will last longer since there's less chance of waste becoming exposed on top of the surface. The disadvantage is that they're harder on older cats who might not be able to climb into them easily or get out when they need help getting up after using the bathroom in it. A shallow litter box may work if you have an older cat that has difficulty climbing in and out of the box.