Oh, and I forgot the crypto price inflation part.
Here is how I see it. Suppose you're at an auction. You bid 10 million for a painting, the guy besides you bids 11. Then you bid 12, and he bids 13. Eventually you get it for 20 million. But then you find out the guy never had any money and he should have never been bidding. You could have gotten it for 10 million, but because of his fake bids, it got to 20 million. Somehow he managed to trick the auction house into making them believe he has the money when he didn't.
Since USDT might just have been printed out of thin air, its easy to use it to buy BTC, and you can print as much as you want. They say they have a dollar for every USDT coin they bring into existence, but if they never received this dollar, then its a scam.
If with just a few million of USDT coins you can move the price of BTC by thousands of dollars in minutes, then billions worth can have moved BTC from 10k to 20k and so on. If you don't have a fake buyer with unlimited fake resources, then what is a BTC coin really worth? People maybe mostly buy BTC because they see it going up. But if it didn't go up because there were no transactions done through fake USDT coins, then maybe the price would have never risen.