In the United States, the theory of Social Darwinism was used in the "Gilded Age" to justify the fact that some people were gaining immense power and wealth while others remained in poverty. This growing stratification of wealth was occurring as a result of industrialization.
The theory of Social Darwinism argues that some people are more "fit" than others to succeed in a competitive economy and society. In this competition, the fittest people rise to the top and the less fit sink. This meant that anyone who was rich during this time had (the theory goes) gotten that way because they were more fit than others. In other words, they were simply better and so they deserved to be rich while the others deserved to be poor. This justified the growing inequality because it said that that inequality was natural and not the result of bad actions on the part of the rich.