There is some controversy as to whether imperial powers actually did benefit from imperialism. Imperialism is sometimes said to have been as much about national pride as about economic or strategic gains.
If imperial powers did benefit from imperialism, it was by A) taking the natural resources of the colonized places and B) by being able to project their military power over long distances. England, for example, gained access to things like cotton and silk from India. It also got opium, which it exported to China, thus helping to pay for the tea that Britain needed from that country. England also made itself militarily strong by having bases in Africa and Asia from which its ships could control a great deal of area.
In these ways, imperial powers (arguably) benefitted economically and militarily from their empires.