1 USDT is virtually equivalent to $1, it is a coin tethered to the US Dollar. So when you sell your BTC, you can store your sale in USDT while you wait for a future transaction. Its the same thing as holding USD, but keeping it in the form of a coin. I don't know anyone who actually buys USDT to hold, your the first person I have come across that talked about the official Tether wallet - but its the perfect coin to use as a temporary store in your exchange until we make the next trade. I hope this helps!