Empire enlarges the area controlled by a state, which arguably has too much power already, or it wouldn't be able to take up overseas intervention and endless war. There are those who make a buck on empire. The liberal approach to imperialism is not the least of these useful ideas. For society, the state-corporate alliance is rather parasitic. Empire, then, is the state writ large.
Sundry motives drive the process.
In the US case, the engrossing of overseas markets has been the abiding goal of empire-builders since 1898. Widespread support for Open Door empire has rested on a desire to remedy perceived economic dilemmas. Wealth and power, as Republican theory tells us, are powerful motives. In all this, theory plays an important, if subordinate, role. One need not be a Marxist to second the motion.