These "robber barons" played a number of roles in the US economy during this time.  Let us look at a few of them.

• Consolidators.  These entrepreneurs were actively involved in creating larger companies.  They used ruthless competition to absorb weaker companies, leading to an economy that was dominated by very large firms.
• Creators of efficiency.  Part of what allowed them to do with was more efficient methods.  They found ways to make their processes as efficient as they could, thus lowering costs.  
• Opponents of unions.  These men tended to be very opposed to unions.  They saw them as a real threat to the capitalist system. This is how a man like Carnegie came to be connected to something like the Homestead Strike and the violence that came out of it.
• Philanthropists.  Through the idea of the "Gospel of Wealth,"
   these men ended up spending tremendous amounts of money helping the poor and other parts of society.  They did things like building libraries and founding universities.  They saw this as a way for them to help improve the society in which they lived.