Knowing what is grey literature and by purposefully searching for this information about your research topic, it can help to provide you with a more comprehensive picture. It becomes necessary to search for this unpublished information due to publication bias. Publication bias refers to when studies that have negative or insignificant results do not get published. As a result, by not having a complete picture, there is a chance that patient safety may be at risk.
Ben Goldacre: Where’s the rest of the data iceberg? April 2012, Washington DC.
The inclusion of grey literature (unpublished studies and such) in systematic reviews is widely recognized as important, and as such, international organizations have incorporated this information in their guidelines and manuals for working on reviews and meta-analyses.