If I may get it right, I am just trying to get use to this whole tether idea and its usage, I thought tether is only to preserve your investment, and since there is not form of increase or decrease going on, you can only use it to store your fund, so even if they pair it with other altcoins, it is not the USDT that is increasing, it is the altcoins itself that is performing the increase, while people change it immediately to USDT to preserve that particular value of the altcoins, instead of keeping it in the coin because of its high fluctuation rate.
Tether USD is something like an insurance stable asset and can be used to determine value of Bitcoin at first, then altcoins. It is a bit strange to see Tether USD has such role in crypto, especially for Bitcoin because it is built on the second layer of Bitcoin network. Today, years after the birthday of Tether USD, it has played solid position in crypto. There are some potential substitutes, but itis hard for them to really replace Tether USD.