Military force projection. To maintain an imperial hegemony, you need some kind of presence in the places you’re looking to control. That control in turn can be used both for economic advantages (for the imperialist, mind you, not for the exploited) and to establish proxies as defense against other imperialists. The former benefits the treasury; the latter national security.

Both fuck over the other countries, but you can guess how much the imperialist powers care about that.