The value of the USD is supported by the US government, most cryptocurrencies are tied to the USD, which makes trading in cryptocurrencies easier for users. This is why there are currencies like USDT.
Tether, the company responsible for issuing USDT in circulation, maintains USD equivalent to USDT value in reserves at all times. Otherwise, the value of a USDT cannot be guaranteed to be equal to one USD. Although this concept has many meanings on paper, things have not been easy for Tether and USDT to date because of the transparency issues.