Study design refers to the methods and methodologies used in research to gather the data needed to explore a specific question.
Some research questions are best approached by statistical analysis of data. This is quantitative research.
Others are better answered by looking for patterns, features or themes in the data. This is qualitative research.
Unreliable research can still be published even with peer review processes.
Effective evaluation of research involves assessing the way a study has been designed and conducted. This helps you determine how valid the research is for your own study or practice.
Being able to identify and understand different study designs means you can spot the signposts of a good or flawed study. Essentially you are using your critical thinking to evaluate evidence, which you need to do in assessments like literature reviews.