If no one believed in bitcoins, or STEEM, it would not have a value allowing you to do purchases, trading or investments. Simple. It is exactly how society have unanimously agreed that some paper printed with fancy ink and numbers displaying $10 holds a value allowing the owner to purchase food, pay rent or go on vacation. Cryptocurrencies hold a real life value because the community creating, advocating and experimenting with them genuinely believe in them being worthy of holding a value allowing you to purchase food, pay rent or go on vacation.