Its not about keeping wealth in USDT. Stable coins like USDT, USDC comes into the picture when someone wants to sell high, wait for dip and buy low. It is obvious that USDT is not one kind of investment but it helps to accumulate wealth one possesses.
If someone want to do that, then why can't they keep the balance in USD or EUR in the exchange itself? What is the point in converting USD to USDT? I know that storing your funds in exchange is a very risky option, and there is a good chance that the exchanges may close down suddenly taking your funds with them. But storing your money in USDT involves the same amount of risk, as it is a token issued by the Bitfinex exchange.