USDT is very useful in my opinion because I often buy USDT if I have sold the altcoins I own. and I also frequently use USDT as my altcoin exchange partner. because, before I bought another altcoin to trade I had held USDT. so I think USDT is the best place to store assets because USDT is a stable altcoin.
Storing asset in form of USDT for a long time definitely a bad idea, tether could always freeze your USDT and a huge chance of exchange poking into your nose if you happen to hold a big amount of USDT in the exchange.
The main idea is to just convert it from more fluctuative asset like decentralized crypto to something equal to USD so you don't have to worry about losing money in the process of withdrawal. Though, there's people who hold USDT for a long time mainly for staking.