Gold is backed by the fact that no one can replicate supernovae which are required to manufacture it, so it must be found. Thus it is scarce. It has several industrial uses and is a source of vanity for many humans, which has inherent value, as humans naturally take pride in adorning themselves. Gold has proven itself over millenia, since before prehistory really, to have value on the open markets. In fact, all rocks do. Gold is hardly unique in this matter. Rocks all have value, but some rocks are rarer than others, thus have more value than others. For example, even sand has value despite its ubiquity. Many companies make their entire profit from collecting and selling sand.

So, in sum, gold can be used in the manufacture of durable, long-lasting goods. It's crucial for certain electronics. There's a constant demand for it for turning into goods. Other rocks can be used for other things, like ground covering, structural support, etc. Bitcoin can be used for?... ? ... ? ... ? nothing