Why do cats do this?

He saying - "come here. I'm hungry."

the same reason deer hunters make grunting noises.

Cats are smart, lol.

I believe they chatter because the prey is moving too fast, or there is some other reason they can't get to it (e.g. being inside, or a bird flying out of reach). Basically, they're frustrated.

I most believe they're just excited (although frustrated also makes sense). Often this will be followed by running around frantically, trying to find another way to get at their prey. Maybe they're thinking, and even saying, "I'm sooooo going to get you..." in full pre-match smacktalk style.

However, I once read something completely different: that when a cat "chatters" (this video isn't the best example of it) at prey from a distance, they are getting their jaw prepared to deliver an instant-kill bite to the prey's neck, should the opportunity or need arise. Pleasant, huh!