It’s always one or all of the following things: They want to feel secure while they rest and or make sure their fur and its smell doesn’t get messed up. This is also why they usually don’t enjoy getting wet, it messes with their fur. Or when someone pets them, human stench offends them so they furiously lick themselves. They also worship the ithyphallic god Min and wish to emulate the function of penes and erect phallic symbols everywhere including your backyard.
One of the above is not true. Maybe more.