what are tails for some creatures have a use but most don't seem to?

Dinosaurs use their tail for balance. Many dinosaurs are bipedal, and they are balanced at their hip joint. That means the weight in front of the hip joint has to be roughly the same as the weight posterior to the hip joint. If a dinosaur evolves a larger head, then its tail region has to evolve to be heavier. T. rex has a huge head, and it cannot balance itself by evolving a massive tail, so it does the next best thing by shrinking its forelimbs to reduce its weight at the front.

Many mammals retain a tail for balance while running, The horse is a good example. However, practically all mammals are quadrupeds, so they are not balanced at the hip joint, and they are more stable than the bipedal dinosaurs, so they can and do shrink their tail, even when they evolve a big heavy head end because of horns and antlers. In these animals, such as deer and cattle, the tail can be a fraction of the size of their head, because it is not needed for balance. Indeed the tails of some quadrupeds can shrink to a vestige. There are some animals that still use their tails. For example, New World monkeys have prehensile tails. They can actually hang onto a branch by the tail alone. It would be wrong to say that New World monkeys have no use for their tails. Another animal that has a definite use for its tail is the snow leopard, which uses its very long tail for balance when it runs swiftly in steep terrain. Needless to say, the rattlesnake has a very definite use for its tail. Many salamanders and lizards also have a use for their tails in self defense. Some salamanders, when they detect danger, will use their tails to distract a predator by moving the tail slowly while the body remains motionless. Many predators will bite on the moving tail and get a mouthful of poison. The tail also comes off easily. The salamander may lose its tail and grow a new one, but it escapes because the predator will not attack after it bites the tail and get a burning mouth sensation. Lizards do not have poisons, but many will readily shed their own tails without being touched. The tail then moves and distracts the predator while the lizard gets away.

As you can see many animals do have a use for their tails. Many animals, however, have little use for their tails. The hippo, for example, has a stubby tail and so do rhinos, because these animals are heavy and well balanced on their 4 legs, and they do not require a tail to maintain balance. Some animals, like the apes, have completely lost their tails, because they evolved brachiation, which is moving around the trees by swinging around the branches with their forelimbs. A tail is not needed and may even get in the way. So, we see all sorts of tail sizes in nature because some animals must have a long tail, but some definitely do not need any. In others, the tail is minimal in size, and scientists call this vestigial. These animals may completely lose their tails in the future, unless of course they help the animal by driving flies away from their rear end, which is often soiled by excrement, a fly attractant.

The dinosaurs, hat tails to balance out their bodies. Without them, they would just fall flat on their faces.

Cows really do need tails to keep flies away. I live on a farm, and the flies are a real pain in the summer. They cover the cow, and keep getting in their eyes.

The only way for the cow to get them off is to use the tail.

We don't have any use for a tail.

A lot of the time its for balance. A dinosaur would've been top heavy - think of the t-rex's big head so walking and running would've been difficult so a tail evened him out. Other animals use a tail for locomotion - think fish swimming or as an extra limb like monkeys. They can also be used as a weapon or as an advert to attract a mate like the peacock.

Well, tails can actually be used as weapons also. Well, for the dinosaurs mainly. I have no clue about animals today using their tails as weapons except for maybe the gator families, sometimes, or barely. But yes, most use their tails as fly swatters, but others, like monkeys, would occasionally use their tails for hanging onto tree branches. Cause I mean, there isn't much that these animals can do with their tails.

Most do have a use. If for nothing else, they usually help with balance and/or steering.