The first thing I would talk about in terms of positive stuff done by the "robber baron" types would be their charitable works.
The most famous of these charitable works, perhaps, was that done by Andrew Carnegie who had gotten so rich in the steel industry. He contributed, for example $60 million (that would be worth a LOT more today) for the building of public libraries (source: The American Pageant).
Second, the industrialization these men led helped make America a lot richer and, eventually, increased the standard of living of the average American. For example, the GDP per capita in the United States in 1870 was $2457 (in 1990 dollars). By 1913 it had more than doubled, reaching $5307. This shows that the US was getting richer during this time. And that was driven by the industrialization. (Source: "History of the American Econom," by Walton and Rockoff)
One of the lasting legacies of the industrialists was to create the reality that making money is not a bad thing. If nothing else, they proved that the desire to make and keep wealth is something that is a vital component of what it means to be "American," and a part of the national identity. The industrialists proved that economic success was vital to American interests, and in their example, another reality of America became evident. The praising of the free market system of economy became lauded during the Gilded Age, where individuals could use capital to generate more of it. Part of what made the Industrialists of this time period so much a part of American History is that they were able to demonstrate that to generate and propitiate a sense of economic rights within one's own sense of being is of critical importance.