Imperialism as conducted by the Great Powers of Europe was multidimensional. The economic dimension was the most obvious and easiest to quantify. It was also connected to mankind's historical drive to explore the unknown. Monarchies across Europe competed with each other by financing expeditions that "discovered" vast regions for colonization and exploitation. From the Roman Empire forward, European regimes sought to explore that which lay far beyond their frontiers and to claim as their own that which they were able to conquer. The benefits were both tangible and intangible. Through colonization, these regimes were able to exploit natural resources from gold to cinnamon as well as man-made goods like silk. At the same time, they were able to expand their power through their hold on territory and the exploitation of resources and slave labor while simultaneously building for themselves vast buffer zones that enhanced their sense of physical security from invading outsiders (such as they had been themselves).
With respect to mankind's drive to explore the unknown, many of history's greatest expeditions were launched at least in part out of an almost messianic fervor to explore. Ferdinand Magellan, among the most successful (until his untimely demise) explorers in European history, noted the following about this innate need to look beyond the known world:
“The sea is dangerous and its storms terrible, but these obstacles have never been sufficient reason to remain ashore... Unlike the mediocre, intrepid spirits seek victory over those things that seem impossible... It is with an iron will that they embark on the most daring of all endeavors... to meet the shadowy future without fear and conquer the unknown.”
So, right off the bat, we have both curiosity and economics as major drivers of imperialism. As competition among European monarchies for the spoils and prestige of overseas colonies heated up, an additional factor emerged. Imperialism, especially that emanating from Western Europe, had a geopolitical dimension that allowed for Marxist-Leninist interpretations of European actions with respect to the latter's colonies. Indeed, at the heart of Lenin's theories of imperialism was the notion that competition among capitalist societies for overseas markets and resources was the main cause of the war then ravaging Europe--in effect, World War I.
The sociocultural factors involved in imperialist policies were also of major relevance. In the development of European culture, Christianity assumed a messianic role that viewed colonization as a form of divine intervention. In other words, "enlightened" Europeans and Christian missionaries would civilize "primitive," "barbaric" peoples. This factor in the practice of imperialism should not be underestimated. As German scholar Benedikt Stuchtey wrote in Colonialism and Imperialism, 1450-1950:
"Even the harshest critics of expansion policies – starting with Bartolomé de las Casas (1474–1566) to the Marxist-Leninist criticism of the 20th century – did not doubt the civilizing mission that justified colonial hegemony."
The notion of forcibly civilizing the uncivilized extended to the US policy of Manifest Destiny. Western expansion across North America certainly had geopolitical and economic dimensions, but there was also a religious or cultural dimension as well. It was considered America's god-given right to expand westward and to settle land as far and wide as possible.
The benefits of imperialism, in closing, were economic, political, and cultural/religious. The sense of destiny, the drive to explore, the desire for commodities, and the denial of the spoils of imperialism to other great powers were all a part of the equation.