Originally, to suppress minorities:
...Leading supporters of legal minimum wages, certainly the most influential economists among them—Ely, Commons, Henry Rogers Seager, Sidney Webb, John B. Andrews, and others—were Progressive reformers, and many were AALL leaders. Progressive-Era marginalists—Alfred Marshall, John Bates Clark, Frank Taussig, Philip Wicksteed, and A. C. Pigou—generally opposed minimum wages (Leonard 2003b).
More surprising than Progressive support for legal minimum wages was the fact that Progressive economists, like their marginalist interlocutors, believed that binding minimum wages would result in job losses. What distinguished supporters of minimum wages from their marginalist opponents was how they regarded minimum-wage-induced job loss. Whereas the marginalists saw disemployment as the principal cost of binding minima, indeed as the reason to oppose minimum-wage legislation, minimum-wage advocates regarded minimum-wage-induced disemployment as a social benefit—a eugenic virtue of legal minimum wages. Sidney and Beatrice Webb ([1897] 1920, 785) state it plainly: “With regard to certain sections of the population [“unemployables”], this unemployment is not a mark of social disease, but actually of social health.”...
The original controversy over the minimum wage centered on what to do about the "unemployable class." By which they meant "black people" and (and, this has always been one of the most memorably bizarre early Progressive phrasings) "the Mongrelized Asian Hordes". It was Syndey Webb (a major early proponent)'s belief, shared by many of the progressive economists affiliated with the American Economic Association, that establishing a minimum wage above the value of the "unemployables" worth would lock them out of the market, accelerating their elimination as a class. "
Of all ways of dealing with these unfortunate parasites," Webb observed, "
the most ruinous to the community is to allow them unrestrainedly to compete as wage earners."
Sociologist E. A. Ross put it succinctly: "
The Coolie cannot outdo the American, but he can underlive him." Since the "inferior races" were content to live closer to a filthy state of nature than the Nordic Man, the savages did not require a civilized wage. Hence if you raised minimum wages to a civilized level, employers wouldn't hire such miscreants, instead preferring "fitter" specimens, making the undesirables less likely to reproduce and, if necessary, easier targets for forced sterilization. Royal Meeker, a Princeton economist and adviser to Woodrow Wilson, explained: "
Better that the state should support the inefficient wholly and prevent the multiplication of the breed than subsidize incompetence and unthrift, enabling them to bring forth more of their kind."
We got the Davis-Bacon Act for the same reason. Darn ole blacks and immigrants were getting jobs when Decent White People' weren't. So, you raise the barrier for entry into the market, knowing that you'll price out many of the "undesirables".