"Backing" is also grossly misunderstood in general.
Any "Backing" a currency has can generally be thought of as an put option on something fungible.
For example, gold will never drop below $10-$20 per Oz because of the industrial/jewelry/etc uses people find for it. However, that "backing" only makes up a small percent of the price of gold.
True; in a sense, its a formalized potlatch system in which the publicly-demonstrated destruction of wealth (energy and wear-and-tear on hardware, demonstrated via computation proofs) is rewarded instead of with informal social regard, with concrete exchangeable tokens.
It's unusual in that it does have a cost of creation - the expense of the machinery and power required to mine a bitcoin. So it does have a "base value" of a sort, you just can't redeem it.