> But who is selling their (temporarily) valuable bitcoin for worthless tether instead of USD?
You're asking what the utility of stablecoins is:
Stablecoins are widely used as a legislatively advantageous on- and off-ramp for the whole crypto market.
Off-ramp: If people want to sell BTC or other volatile tokens because they want to realize profits, the obvious thing would be to sell for fiat (USD/EUR). But due to taxation and legislation this is often difficult. Stablecoins like Tether provide the utility of a low-volatility currency that fiat would fill. Main advantage is bypassing legislation.
This use is so common there is jargon for "Tethering up".
There also exist many debit cards that allow paying with stablecoins, increasing utility. The rise of DeFi and money markets for stablecoins also provides a good return on stablecoins while in theory being low-volatility.
On-ramp: If people want to buy a certain token they first go into Tether. This is usually for bypassing local legislation limiting the buying of a specific token or use of exchanges. This use is less common, I would wager.
Of course, for any of this to be actually useful Tether has to remain the same price. If Tether starts to deviate significantly from the 1USD peg, a liquidity crisis cuold be triggered.
Personally, I haven't trusted Tether since the first BitFinex scandals and avoid it like the plague. For my stablecoin needs, I use something that is audited and over-collateralized like DAIv2.