The physical 12-volume set is quite large and can be expensive to collect. Digital formats like PDFs or E-books are popular because:
Early drafts of The Fellowship of the Ring , where "Strider" was a hobbit named Trotter. the history of middle earth volumes 1-12 pdf
Deep philosophical writings on the nature of evil and the "Athrabeth Finrod ah Andreth." The physical 12-volume set is quite large and
The final stages of the War of the Ring and the "Notion Club Papers." The Later Silmarillion and Appendices (Volumes 10-12) the history of middle earth volumes 1-12 pdf
Early legends of Beren and Lúthien and the Fall of Gondolin.