Very few economic transactions are denominated in gold, but having it is a hedge for the contingency of finding yourself in a situation where you can only do a transaction in gold, and then it can be life-saving (bribing your way out of a European country in 1944, etc.)

The reason people believe gold will retain meaningful value in a situation where nothing else does is thousands of years of history where it did work out like that. It's debatable (to say the least) whether Bitcoin with its mere twelve years of history and inherent dependency on easily disrupted digital networks could have the same properties.