Tails are something that most animals have. They had different uses for their tails depending on the species, but for the most part, it's a feature that's unique to animals.
For many young, including lion cubs, the tail is a plaything. Particularly, Uncle Scar's tail is a plaything.
When he slept, he could feel his tail being squished underneath small paws. When he ate, his tail would lift without his control. When he groomed, his nephew would play tag with it.
It made him want to bite off his tail, but then he'd have permanent issues with balance and depth-perception. He'd also look terribly ugly without it. He'd seen lions who lost their tails in battle or some other violent event and they looked stupid. No, he would never look stupid.
Currently, Simba was pawing at his uncle's flicking tail as the dark lion stared off into the distance.
"You do realize that you have a tail of your own, don't you?"
"Of course I do, but yours is easier to catch!"
