In principle, being the preferred world currency and reserve currency means that Treasury bonds are a safe way to build their reserves while gaining interest. This keeps the American dollar and the American led world relevant. In other words, if America goes down, so will you. This also gives the US a lot of coersive leverage over developing nations to exploit their land and labor. This means that the core of the US empire gets access to cheap resources and labor. What the US gets is highly malleable power.