Why do parrots copy sounds?

This is a popular movie gag that someone will say something and a parrot will repeat it. However, it's not guaranteed a parrot will repeat something after you. An African Grey is most likely to do this the first or second time it hears something, as long as it is said with great enthusiasm. Great Enthusiasm, you say? Why would you need great enthusiasm? Well, see here, parrots LOVE attention. They adore it. They live for it. So when you say something with great happiness or enthusiasm a couple of times or so, the parrot will start to think, Hey, whatever this person is saying is of great importance. If I could somehow get that kind of attention... Eventually the parrot may or may not leanr to say it, depending on the bird itself, and it will obliviously get attention for it, as talking is one of the charms of parrots and people love it.
Parrots can also be taught to speak on cue or speak with clear understanding of the Word. Not simply mind-less repetition, but knowing that a grape is a grape and that green is green. They know what the Word means and can use it properly.