Why cats use a litterbox?

Small feral cats bury their feces to hide their trails from larger cats and other predators. Because it is easier to bury excrement in soft dirt, cats naturally seek out a sandy area to defecate. Burying feces may also be a sign of submission by smaller cats to larger cats in the wild. Most indoor cats bury their waste, possibly to display subordination to their humans.
Sometimes in multi-cat households, however, the dominant cat will leave waste uncovered to indicate his status.

Kitty Litter Cake!