ROG tend to have more "gamer" oriented features... They would come with above average on board audio, RGB galore (and Asus AURA), typically more fan headers, better included software for various things (I believe one of these things is a RAM DISK software in case you wished to play around with that) and so on. They tend to come with better RAM overclocking compared to the other Asus Boards. Their on board RGB headers in the Z270 series are RGBW headers, thats about the only truly unique thing they have going atm as far as i know.
However, there isn't really anything all that special or unique about ROG boards. you can find most of these features on a lot of boards and companies out there.