Answer:
Economic development.
Step-by-step explanation:
Imperialism is a method that a country use to expand its power by using force/military power. It's very hard to take a positive effect from this from the perspective of the one that's conquered.
But, from the perspective of the conqueror, it's easy to determine the positive effect.
Through military's forces, the stronger country could take over all the resources that exist in the conquered area without having to pay a dime for it. This will massively improve their economy even though it will destroy its victim at the same time.