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Tether (USDT) doesn't get much attention from investors looking for profits, since it's more of a utility coin than an actual investment - It's only used for stability while switching between one investment to the next. This is also why we see such a massive trade volume relative to the total supply - Very few traders are holding this coin since there's not much potential for profit, but many people are using it as an intermediary between other currencies.

On any given day, the trade volume of Tether is nearly twice its market cap:

Tether (USDT) is also controversial since there's no easy way for them to prove that they have enough fiat currency to back up the USDT that they've issued without revealing where their money is being stored. Revealing their bank accounts/financial backers would present a huge risk for them if a government decides to attempt to force regulations onto cryptocurrencies. However, as long as the majority of stakeholders continue to treat 1 USDT = 1 USD, it still serves as a viable medium of exchange, regardless of Tether's actual financial state.

This is very similar to US dollars - If everyone tried to withdraw their physical cash from the banks at once, the banks would quickly run out of physical currency to represent the value. However, since only a small fraction of the currency is being represented in cash at one time, the system continues to function.