[toast0]

You don't need ARC to address a/v sync. HDMI has (optional) metadata somewhere (edid?) where the display device indicates its delay and the audio device can adjust accordingly. It's helpful if the display device has fixed delay for this feature to be most effective; it's fine if there's different modes with different delays and the current delay is communicated, but some modes have variable delay depending on content which is terrible in several ways.

IMHO, ARC is primarily useful when the display device is also acquiring the content: it's running the TV tuner or internet streaming or content off a usb drive. It's also useful if you have a 1080p capable receiver and upgrade to a 2160p(4k) display and sources: if you put the receiver in the middle, you lose on video quality, but with eARC the display can route full quality audio from all your sources. Some sources do have two HDMI outs, so you could wire to the display and the receiver, but that's not very common.