Before getting into the nitty-gritty of cryptocurrencies, you need to understand the definition of money itself. The philosophy behind money is a bit like “which came first: the chicken or the egg?" thing. In order for money to be valuable, it must have a number of characteristics, such as the following:
Enough people must have it.
Merchants must accept it as a form of payment.
Society must trust that it’s valuable and that it will remain valuable in the future.
Of course, in the old days, when you traded your chicken for shoes, the values of the exchanged materials was obvious. But when coins, cash, and credit cards came into play, the definition of money and, more importantly, the trust model of money changed.