The killer feature I like for Arc is that I can sync my vertical tabs across windows. I use two monitors and sometimes switch between the windows depending on what I'm contextually doing.
This is primarily for users with multiple displays. Both displays contain the same tabs and switching between them saves the state. What's also nice is this state matches my laptop, which only has one monitor. So between my desktop with two and my laptop with one, its a single window/browser state.
