For me it is a question of depth and breadth of engagement to his life's work. For some, the purely narrative
books are enough. For others there is a desire to read every word written by JRRT or CT about JRRT's writings.
Though the HoME books may seem repetitive, confusing, dense and incomplete by some readers (which is a fair assessment) many others, myself included, find every offering even the earliest and fragmentary ones beneficial to understanding the whole. In addition, it's
Tolkien for
Eru's sake! We are simply compelled to lap up every ink drop from Tolkien's pen. (Excuse the melodrama.)
And as SES touched upon, as an amateur writer I love watching over Tolkien's shoulder as he writes, edits, re-writes, scratches out, makes additions, changes his mind, makes up his mind endlessly. He was so brilliant it is impossible for me to ignore his own commitment to his craft. I only wish there was more.