1. Not only did the Populist Party strengthen the Democrat Party, but also helped shape the Democrat & Republican platform in favor of the people (to obtain votes).
2. The demand of free coinage of silver made the U.S. government adopt silver as the legal standard, which lead to the Sherman Silver Act.
3. Helped turn the attention of the other parties to other problems. Ex: The 17th amendment provides for a direct, popular election of senators. This amendment was passed after the Populist Party brought it up.
hope that helps