The research design stage is the most important of all the stages in a research project. It is the stage that does the most to determine how well a project will go.
Research design sets up a plan for the whole project. It determines what questions will be studied. It determines how data will be collected and which data are important enough to collect. It then determines how the data will be analyzed.
This contributes to a successful outcome by setting up a good project that will actually help the researchers answer their original questions. Without a plan, the researchers are simply flailing around aimlessly. With a bad plan, they are doing worse; they are going out and collecting data that will not help them to understand what they are trying to understand.
Since the research design is a "road map" for the entire project, it can contribute mightily to a successful outcome.