Many animals on land have a tail, even though they evolved in different lines of descent like reptiles and mammals did.
Signalling to each other is one function (see dogs for example). Balance, especially in trees, is another. Another thing to think about is the change from dinosaur tails to bird tails. Bird tails directly steer flying. They also have a signalling function. Don't forget insect tails. In flying insects they do help steering, much as bird tails do. In fish it's all about propulsion.
Humans don't have a tail. That puts us in a minority of simians which are tailless. It's not obvious why, though it perhaps has something to do with our upright stance. Unlike monkeys, apes have no tail. It's certainly seems easier to sit down with no tail, but monkeys manage with tails pretty well.
Some species of lizard can detach (drop off) their tails from their bodies. This can help them to escape predators. The predator is either distracted by the wriggling, detached tail or left with only the tail while the lizard escapes.
- Byron Dawson 2003. The Heinemann Science Scheme. Heinemann, p125. ISBN 978-0-435-58332-3