We now know that Grey Literature is the opposite of Scholarly, Peer-Reviewed literature. Unlike Peer-Reviewed literature, Grey Literature is not reviewed by experts before being published, which means readers have to do all the work of evaluating the literature themselves. Why would someone use Grey Literature when it is generally less reliable and requires more evaluation on the users' part than Peer-Reviewed Literature? Among other reasons, Grey Literature is often...
..than scholarly, peer-reviewed research. Keep in mind that no matter the reason for using grey literature, you will need to properly evaluate it.
Universities and other academic institutions have a diversity problem and that is reflected in the scholarly and peer-reviewed literature that is published. Historically, scholarly literature was made by white, western-centric, affluent men because that is who could access the academic institutions that produce such literature. Diversity efforts have changed this to some extent, but it is a major problem. This means that scholarly literature is often missing important viewpoints and information that could be extremely valuable for building knowledge. Grey literature does not require authors to have made it past all the barriers and hurdles of academia so it is often more diverse than scholarly literature.