You hold it if you want to use the money to trade crypto, because banks really don't like you moving around large amounts of money.

So you hold USDT in a wallet instead of transferring it back to the bank. It's also a lot faster to transfer money using stablecoins then it is with banks.

If your regular bank account is in USD that thinking is reasonable. But if your bank account is in another currency you might hold tether to protect yourself against hyperinflation or other things you can't control in your home currency.

Someone must be holding them unless they are popping in and out of existence.