1. A lot of newly 'minted' Tether's are being used to buy Bitcoin (so prop it up, the Bitcoin price rises).
2. Possibly as a next step, these now more valuable bitcoins are than sold for real money, and the owners of bitfinex/tether pocket this real earned money as profit.
3. It should be noted that although the total amount of tether is still rising, the total amount of all "stable coins" is rising a lot faster, so basically, outpacing the growth of tether: the market seems to be on to them, and puts more trust in other stable coins. Personally, I like Maker's DAO decentralised SAI stable coin a lot. But preferences vary.
4. Why is there any demand at all for any stable coin? Can't a person just send/receive real dollars/euros? Answer: No .
There are a lot of unbanked people in the world, more than we in our western minds would dare to guess, I think.
Look at El Salvador: in 6 weeks since officially adopting Bitcoin as a currency (next to the US dollar, not replacing it mind you) the number of people that now have "wallet" software running on their phone is already lager then the number of people who over the past 20 years were able to open a bank account.