Civilians exist in Empire to work and provide cash to the government. Simply by existing, they generate revenue to their host country in proportion to the efficiency of the sector in which they work.
The military-industrial complex exists in order to siphon off hard-earned tax dollars. Military cost (in general) about ten times as much as a civilian pays in taxes.
In addition, final goods and services cost money to produce. Ships, land units, planes, guns and shells are all examples of the costs inflicted upon a helpless government by the military-industrial complex. Also, services such as technology research, medical research, and even happiness (money and lcm's can buy happiness here...) cost something to create. Consumer items such as food, lcm, hcm, and all other intermediate goods (or raw materials) cost nothing other than the hard work of the oppressed citizenry.
Note that it takes ten civilians to keep one soldier in the field. Note also that the only way to make money is either to be pacifistic and let your nation accumulate wealth by having a small standing army, or by selling products to other nations. Also, land units can multiply the strength of each mil, making them perhaps more economical, if you already have made the technological investment.
Also note that captured civilians only pay 1/4 the normal taxes. They like to hide money until they are all finally agree to become naturalized citizens of their new government.