By definition, grey literature are materials published by organizations whose purpose is not publishing. So, they constitute a great bulk of information that is hardly accessible, yet entirely pertinent to particular areas of research. Think of government reports that are not published consistently, manuscripts from defunct organizations, laboratory reports, trade unions, trusts, universities; these all encapsulate grey literature. As such, since it is so particular, it tends to be highly relevant to particular forms of research. To rely solely on academically published materials (articles, journals) is silly for a researcher who wants to research particular studies that never fit into an article narrative.
So, there you are. Librarians froth at the bit at this type of stuff. The image was taken from a feasibility study on mobility scooters on public transport. The full report can be found here, if you want to know if scooters are feasible. I am not sure this link will see the most action, but in the interest of full disclosure….