Why buy USDT instead of USD?

I'm a bit confused about the difference between USDT and USD. Can you explain why someone might choose to buy USDT instead of USD? I've heard about the concept of stablecoins, but I'm not quite sure how USDT fits into the picture. Is it safer? More convenient? Or does it offer some unique benefits that USD doesn't? I'm interested in understanding the reasons behind this choice, and how it might impact my financial decisions.