Using USDT is more stable than BNB, and it's easier to do calculus with a tether. If you buy with BNB and the price goes up then you will get less altcoins, using USDT is more stable at least in one fair.
That's clear and the pairs with USDT are also more so that it doesn't get complicated when everyone wants to choose a particular coin through the USDT pair, it will also be easier to measure and estimate well by the trader himself with the USDT measure