You can't walk to a bank and trade 1btc for X amount of gold, though. It cannot be traded for something else of value at a fixed price.
If it isn't necessary for survival, then any "value" we assign it is arbitrary.
What is gold backed by? There is no guarantee that I can walk in to a bank and exchange my gold for something of value.
Gold has an arbitrary value because people want to own it and it is rare.
Bitcoin has an arbitrary value because people want to own it and it is rare.
However, now USD and all(?) fiat currencies are worth the amount of trust ppl have in them, the same as with BTC.
Same with gold. The value gold has is due to the trust people have in it.