The value of money is founded on the premise that people accept it as payment for goods and services. If no one takes your paper money, maybe you could start your BBQ with it, but that's about it. If it is not intrinsically precious, if it is not actually made of something valuable, the only value money has is that it is an accepted medium of exchange.
Why do we accept something with no intrinsic worth in payment for goods and services? Because others must accept it from us. Governments require entities over which they have control to accept their currencies as legal tender.
So-called cryptocurrencies enjoy no such government mandate. Nor do they have a treasury department, as nations do, controlling the amount of currency in circulation: the US Treasury, for example, sets the funds rate on the reserves commercial banks deposit with it and sets the interest rate on so-called "reverse repo" transactions in which it borrows money from private commercial institutions. These actions are meant to increase or decrease the amount of money in circulation. A cryptocurrency caps its supply instead; however, it is subject to dilution by competing cryptocurrencies.
Cryptocurrencies are not backed or collateralized by anything, but neither are currencies like the US Dollar, which has not been backed by gold since 1934. US Treasury notes are backed by "lawful money".
All of these considerations combined are the premise of a currency, the foundation on which its value rests.