Some of these categories have many books or e-books under that listing. For example, Historical Fiction has over 22,000 offerings, probably more than the entire collection of some public libraries. It can be difficult for readers to sort through the offerings so in that way further specification could be useful. Now, if you know exactly what you are looking for you can type that in the search box. This feature would also work if you can't remember the title or author but remember and can describe some aspects. A very little can suffice. I know I am bored when I try to see if I can get a particular book to come up based on some aspect.
For example, if I remember the opening line from Pride and Prejudice which advises that a single man of large fortune must be in want of a wife, I get some unusual suggestions but one does give the title, if not the exact Jane Austen work. If I enter Mordor assuming this is the only aspect of Lord of the Rings (either the movie or the book trilogy) that I recall, I will be shown a Lego Lord of the Rings mini Mordor Orc figure. Who knew? But now I have the title.
Eventually the game deteriorates to looking for one of my books through various and obscure means. I'm often impressed how Amazon's no doubt mighty computer system can ferret titles out from small clues. So, 'Surrey restaurant Mystery' does cause Death at Table 15 to come up and 'Dental assistant Mystery' does call up Operatory of Death. Most amazing of all to me, the mere entry of the pseudo-word 'Reconstitutionalists', the oppressive government in A New Premise, brings up that title and that title alone. But now I'm beginning to wonder if the said computer has become wise to me.
Sometimes authors must seek new cures for writer's block!