As someone who runs a market making bot, tether is crucial because so many pairs are denominated in tether.
I'm a small player, so it's easy and not very expensive to borrow the tether if you don't trust it (which I don't), but if you are making making on a large scale is probably too expensive to borrow.
The other possibility that occurs to me is that if you need tether to succeed because you are invested in its success, you might hold large amounts to control the float.
Note that there are only 4 accounts that have more than a billion tether: https://www.coincarp.com/currencies/tether/richlist/
It seems to me like holding billions in USDT is very risky, so no self-interested actor should do so, yet we find billions of dollars in USDT out there. Who holds them and why?