The theory is that Bitfinex creates these out of thin air and buys other cryptos with them, thus raising the prices across the board (the prices quickly bleeds into all the other exchanges because arb). Crypto holders buy tethers when they want to short crypto against "USD" since tethers are supposedly equal to usd. Often times the entire market swings at once and lots of it dumps into usdt during downturns ("tether up!"), and back again into crypto on an upswing. Anyway, if you look at Bitfinex, you'll notice they have pairs like BTC/USD... except that USD is really USDT. It's basically counterfeit usd and it seems like all exchanges that can't trade actual usd use it
Right now its almost like tether is like the central bank of crypto, constantly bailing out the market during downturns and pumping the price up. They've really upped their printing the past few days due to the massive downturn.