In theory, ARC is supposed to let you have one connection between your TV and whatever you use to create sound: a receiver or a sound bar .
You send video to the TV with an HDMI cable, and that same HDMI cable sends audio from the TV back down the same cable.
This is great for smart TVs,
the audio gets sent to the receiver/sound bar for a massive improvement in sound quality.
Another way this is great is if you have multiple sources (Blu-ray, cable box, Roku),but only one input on your sound bar.
With ARC, you can connect everything to the TV, and the TV will send audio down an HDMI cable to the sound bar. No extra cables needed (i.e. an optical cable).
Lastly, it can allow for one remote to turn on everything in a system. Turn on your TV, your sound bar turns on as well. That's handy.