First of all, let us note that historians typically do not talk about populism as a major force in US society after 1900. The time from 1900 to 1920 is typically described as the Progressive Era. We talk about the populists as a force in the late 1800s. But we will come back to this issue later in this answer.
In the late 1800s, the Populists were seen as the major force that was trying to help American farmers and, to some degree, American workers. The Populists did not have a tremendous amount of success in actually helping farmers and workers, but they did achieve a few things.
One of the first things that they accomplished was to pass “Granger Laws” to help farmers in five states. These laws were meant mainly to prevent railroads from charging rates that hurt farmers. The laws were not terribly effective, but they did lead to the Supreme Court case of Munn v. Illinois. In this case, the Supreme Court held that regulation of railroads was constitutional because the railroads were serving in the public interest even though they were private property. This case helped to promote the idea that the government had the right to regulate businesses, an idea that was to be very important to the American people.
The Populists did not achieve very much more during the late 1800s. They were simply not strong enough by themselves to fight against the power of the big businesses and of conservatives in general. This was to change in the early 1900s.
In the early 1900s, the US moved from populism to progressivism. To some, though, there was not much difference between the two. William Allen White, a progressive leader from Kansas, said that progressivism was really just populism that
…shaved its whiskers, washed its shirt, put on a derby, and moved up into the middle class.
In other words, the progressives stood for many of the same things that the populists had stood for. However, the progressives were more of a middle class, respectable movement and therefore had more success than the populists, who came more from the poorer classes of society.
If we say that the progressives were similar to the populists, we see populism helping Americans in many ways from 1900 to 1920. During this time, all sorts of laws were passed that (most people would argue) helped Americans. There were more laws to regulate railroads. There was tough enforcement of antitrust laws so that monopolies could not abuse Americans as easily. There was a more even-handed approach to labor unions on the part of the government. There were laws passed to prevent companies from selling impure drugs or food that had not been inspected by government inspectors. There were laws that allowed Americans to have more of a say in their democracy through things like initiatives and referendums.
So, we can say that populism, (along with its close relation, progressivism) did a great deal to help Americans, particularly in the early 20th century. Conservatives might not agree, but many people would say that the populists were helpful because they pushed hard for a new system in which the government intervened more in the economy to ensure that businesses were not able to abuse the people.