Bitfnex needs USD for payments, but they cannot print USD. Printing a lot of USDT to buy BTC and liquidating them to USD help them do that. Only when they are investigated for deposit, will they sell the BTC for USD to prove it.
have you checked the supply of Tether ever before? i don't think so. i suggest you take a look at it. then check out bitcoin price. if they were really buying bitcoin with USDT then selling it for USD then bitcoin price should have fallen to $100 and less due to the sheer amount of sell pressure
![Cheesy](https://bitcointalk.org/Smileys/default/cheesy.gif)
Of course it does not sell off in 1 time. 10m usdt is printed to buy btc for 6k, then sell btc for 8k. After returning 10m usdt, the excessed amount of usd is the received interest.