Exchanges with actual USD pairings generally have low volume (Kraken, on account of its terrible engine) or few coins listed (gdax, gemini). So if you want to hedge in fiat on a higher volume exchange like bitfinex or binance and also want to trade alts, you will need tether.

The price of tether will likely never reach 1.1 unless there are solvency concerns. Any price disparity between BtC/usdt and BTC/USD has historically disappeared, so you can pseudo-arbitrage these pairs across exchanges even though they are technically not the same. Kraken's USD/USDT pair also makes this bridge easier.