I'd say that, generally, imperialist motivation is a matter of gaining power. In a capitalist system, capital is power, so they are seeking capital.

The way I explained it was meant to break it down into a modern context to help answer the question, not to address imperialism in the context of feudalism or other systems. End of the day, someone is exploiting someone else for their own gain. It was just a matter of the context of the question and I erred on the side of keeping the scope within capitalism.