Tether should introduce the cryptocurrency community to the concept of systemic risk: after something is infrastructure, it gets into everything. When you withdrawal the infrastructure suddenly, a bunch of things break all at once, even ones that don’t look exposed, because of the transitive nature of the dependency graph.
Where is Tether critical, load-bearing infrastructure? Lots of places. It represents of the liquidity for many “altcoins” (cryptocurrencies which are less popular than the main ones, like Bitcoin/Ether). It is the unit of account and the internal reserve of many of the less-banked exchanges. It very probably functions as the credit supply which keeps liquidity in the system.