Do I need to buy USDT first?

I'm a bit confused about the process of cryptocurrency trading. Do I necessarily have to purchase USDT first before engaging in any transactions? I've heard of it being used as a stablecoin, but is it mandatory for all crypto dealings? Or are there other options available that I could consider? Could you clarify the role of USDT in the larger cryptocurrency ecosystem and whether it's an absolute requirement for beginners like me? Your insights would be greatly appreciated.