Placing a bet of a small amount about half a mbtc then winning seemed to have given me one.
As long as you win a specific amount which is above the criteria to be listed on the high roller tab then you'll get the icon. I got it many times already although my bets were just small amount, but I hit high multiplier. So your bet amount is not the key, but your winning amount is the one that makes you get the icon.
No direct benefits of getting the icon, but if you can get it many times then I believe you'll get surprise reward like what I and other players got some time ago. I received free $100 for being high roller not long time ago, so maybe they will give similar thing in the future who knows. I myself now prefer to use hidden feature so I wont get the icon, as most of the time I get the icon then there were many users asks to be friend and if I accept it, then most of them will only ask for some small amount of satoshi (beggars).
I have received it 3x times now so who knows I might see what you did as a reward if I keep playing larger amounts and win. But of course there are chances of losing the same amount you get as a reward. You know.
Thank you for the other ones who answered my question about this as well cause I just started using the site and know there are ones who have been with them from the very start and know all about these little things most of the newer players would not.