To explain more, for e.g., we hear that the USD was backed by the convertibility of this currency to precious metals until 1971. Now, what are altcoins backed by such as Ethereum or Litecoin? Are mathematical calculations performed by ASIC miners any useful in the real world?
USD was backed by gold in the early days when the federal reserve was founded, but this is no longer the case. Money (fiat) is now backed by nothing that's why we have inflation rising in so many country even in the united states as of today. Altcoins are backed by nothing but what the community says it is, which is way more better than what the government say it is.
Moreover in realife, we do not have any back up for everything like some collectibles, for example. Collectibles are gaining its value from the demand. Similarly altcoins are getting their value from open markets based on supply and demand mechanism.