Social Darwinism was one of the motives for imperialism, it was a political reason.
Social Darwinism- "survival of the fittest" - justified the conquest of weak areas by stronger, more civilized ones
other reasons for imperialism include;
economic reasons- land, natural reasorces (exploited the natives)
social + cultural reasons - missionary work
political reasons- non-whites recive 'the blessing of western society' -law, medicine, education, technology ...