USDT is a stable currency which gives its holder the guarantee of the US dollar. It has two major interests in my opinion:
Its first interest is to allow cryptocurrency exchange platforms to have access to the US dollar, without having to manage real dollars. This is a huge advantage for trading platforms that only manage cryptocurrencies. The second interest of Tether USD is to offer users a way to protect themselves against the volatility of the crypto market.
It is precisely because of the volatility of the market that I mainly use stablecoins but I prefer DAI algorithmic stablecoins for example because there is a controversial garande around tether. The problem with DAI is that there aren't many pairs so you have to go through USDT. I am suspicious of Tether, although it is the most used on the market.
Remember that holding USDT comes down to trusting the centralized player that is the Tether company - for that alone, I will not hold it for the long term.