You sell BTC to USDT if you are not looking to cash out of BTC. Instead, if you think that BTC is at a peak in the short term and you simply want to sell at the peak and get back in once it drops, selling to USDT is just fine. That way you can buy back BTC once price drops.
When you do want to cash out to real fiat, you have to sell to USD.
USDT allows a large number of exchanges to allow you to sell BTC and hold in something that looks like USD. Without USDT a number of exchanges won't see anywhere near the volume they get. By enabling a larger volume, USDT is indirectly helping inflate BTC (and other currencies) price.