Q: What is the importance of using unpublished data as a secondary data source?
Unpublished data or literature is known as grey literature in research. (The term ‘grey literature’ also includes data published in a non-commercial form, such as a conference proceeding.) Unpublished data is useful mainly in secondary research, such as literature reviews and systematic reviews. It provides pointers to new research and perhaps also research paths to avoid. Preprints are a growing form of unpublished data these days and have proved very useful in guiding research in critical areas such as COVID-19.
You may read more about preprints here: The role of preprints in research dissemination
You may also find this previous query relevant: What are the limitations of secondary data collection?
And if you’re planning a secondary research yourself, you may find this piece useful: Secondary research – the basics of narrative reviews, systematic reviews, and meta-analysis
Hope that helps. And all the best for your study!