When cats are kittens, they meow at their mother. Those tiny squeaks are made for a variety of reasons—to let mom know they're hungry, cold, or lost, for starters. But as they grow up, cats stop conversing with one another (adorable YouTube videos notwithstanding). With their inside voices, anyway—the deafening caterwaul of a cat in heat is still targeted at their own species.
Once cats are adults, they only meow at humans. The reason is probably obvious: it's to get what they want. Whether that's food, an open bathroom door, or some ear scritches, those demanding meows are precisely engineered to pull at your heartstrings and help them get their way.