They print tether so it doesn't sky rocket in value. They artificially inflate it to keep the price down (in case of a crash). People exit crypto through USDT which creates demand.
USD -> USDT (new people joining) and CRYPTO -> USDT (margin trading, going against the dollar not btc) rises USDT's value so they have to inflate it somehow - by printing more USDT.
USDT -> USD (people cashing out) and USDT -> CRYPTO (people going back to crypto) deflates its value.
There are 2 ways in which USDT raises its value (against the dollar) - new people joining crypto and people that are margin trading, but only one way which lowers its value (USDT -> CRYPTO) because people are selling USDT in favor of some crypto coin. Only in case of USDT->USD (people cashing out) should the market cap of USDT be lowered, but USDT -> CRYPTO will lower its value because people are selling USDT in favor of some crypto coin.