I believe the hypothesis is that they evolved as a "thing domestic cats can do" for communication with humans, but adult cats that know how to meow, don't just use their meows to communicate with humans. They see it as a general communication tool, and will also try to use it with other cats.

The supporting evidence for this hypothesis, I believe, is that meowing has been observed to be a learned communication technique gained only if a cat interacts with humans during development; adult feral cats, living only around other cats during their development, never learn this technique or its social meaning, and so don't understand the social signal being communicated by domestic cats when the domestic cat meows at them, nor do they try to use it in response.

(Same goes with the domestic cat's social signal of keeping their tail raised when standing to express trust. Feral cats don't develop that habit, either.)