If you're into trading a well designed usd token allows traders to go in and out of btc and usd, without requiring an on/off ramp into the fiat world.

Many people in poorer nations can't send/receive real usd so a proxy is very helpful to them.

A well designed usd token must have 1 to 1 backing. I can't comment if they all do that

Not from me! I've been calling bullshit on Tether since 2016!