I just have got this item but i don't know how to use it.
Everything is said in the description : Sell = 666 666 Golds xD
It's a fancy gold chest. I'm still open to the idea of them having another use so I tend to hold onto them unless I'm totally broke.
That’s actually a great idea!
Well, in a normal game it would probably increase 0.01% chance to get 5* units for every coin you possess from the gold banner or something. But for a mobile gacha I think the description is just flavor text.
I'm keeping mine too. Not sure if it will increase summon rate, but I think I have been having better luck since I had a few of these coins. So it's at least a good luck charm to me.