The industrial revolution and economic motives were almost at the center of imperialism. Those who had a good manufacturing base were able to produce weapons far superior to those who were throwing spears at them. They could produce better ships than those riding canoes. They could produce men better trained in the art of war, who were disciplined and fearless in battle.
The best colonies were those that had what appeared to be unlimited natural resources that were untapped. The imperialist countries could then take the natural resources and turn them into manufactured goods that could be sold back to the colonies under their rule.