The sample methodology in a research paper provides the information to show that the research is valid. It must tell what was done to answer the research question and how the research was done.
Explaining the sample methodology also justifies the experimental design. Because research is orderly and direct, the samples methods section needs to tell what materials were used in the study, describe the research protocol, explain any statistical data and justify why the researcher chose a particular method of research. The methodology needs to state what problems were anticipated, if those problems arose and how those problems were mitigated, if they were. The methodology must give a full account of how data was collected or generated along with how it was analyzed.
The importance of explaining research methodology is so readers do not need to read data themselves. They understand the data based on what the research paper says. It also tells how accurate the data likely is so that the reader knows whether or not to trust it. If recommendations are made, the reader needs to know if he can trust such recommendations. For instance, if a large enough sample size was not used, it is more difficult to trust the results.