There are lots of reasons why different animals have tails, and each reason is specific to how that animal uses its tail!
Think about some animals who have tails, and some ways those animals might use their tails. Fish, for example, swish their tails back and forth to move through the water. Cats and squirrels use their tails for movement in a different way: The tails of these animals help them with balance! Dogs are well-known to use their tails for communication – their tails can tell you things like when they are happy (wagging tail) or sad (tail is down). Foxes sometimes use their tails for extra warmth by covering up their bodies with their tails!
Some other animals with tails use them for protection or self-defense, like kangaroos and crocodiles who can hit an attacker very hard with the strength of their tails, or porcupines who can use their quills. Monkeys also have a special use for their tails. If you’ve ever seen monkeys on TV or at the zoo, then you might already know that monkeys use their tails to help them grab onto things and climb up trees!