Economic theory tells us that American citizens would only benefit from "buying American" if the American companies are able to compete on even terms with their foreign competitors in a system of free trade. If American citizens buy American just for the sake of supporting American companies, they are not helping themselves in economic terms. This is because they are supporting companies that make products at a higher price and/or a lower quality than other companies. This does not help American consumers.
It is possible, however, to argue that buying American helps American citizens in intangible ways. It may help them to feel better about themselves and their country. This could be seen as a benefit of buying American, but it is not an easily measured sort of a benefit.