When an animal has a tail, it usually uses it for a purpose. Alligators use theirs to help them swim. Squirrels and monkeys use theirs for balance so they don't fall out of trees.
Since people walk, and we're really good at balancing on two legs, when most other animals need four, we don't really have any use for a tail! Having a tail would actually throw us off balance, causing us to tip backward!
People still do have a tailbone though. It's a bone at the bottom of the spine where a tail would be if you had one. It usually doesn't stick out, so it looks like we don't have a tail. It's very rare, but every now and then a human will be born with a tail! It's a leftover from our ancestors who probably used their tails to balance. They didn't stand up as straight and balance as well as we do.
Think about all the animals that you can think of that have tails. There will be a lot of them. Think about how they use their tails. A scorpion has a type of tail. How does it use it? Birds have tail feathers. How are those used? A tail that can grab on to things is called a prehensile tail. Imagine you had one of those. You could use it to hang on to branches! Or maybe even pick things up! How would you like having a tail?