Cats have special papillae or spikes on their tongue for the purpose of grooming their fur, smoothing it out, removing dust and loose fur, and moistening their coat. Here is a guideline to help your cat pass a hairball.
The truth of the matter is there isn't one clear answer. However, many experts agree that they meow most often because they want something. For example, they might be asking for some food, or to be let outside, or even to cuddle.