The question is not "Why do some people index things?" The answer is partly "because they are there" and partly "because they might as well NOT be there if people can't find them."
I think it's one of those things that's hardwired into (some) human minds. An pathogenic society funnels those minds into compiling dossiers on politically suspect kindergarteners, or collecting baseball cards, or rearranging dishes in china cabinets in antiseptic parlors.
A successful society invents librarians.
This isn't the only such uncommon skill that's (potentially) valuable, (potentially) pervertible, and easy enough to waste. I have no ear or talent for poetry -- I can't imagine what I'm missing. And, of course, some societies foster epic poems, while others subsidize radio commercials.
You poets: don't try to explain to me what you do, or why you do it, or why anyone should care whether it's done, or what it means for it to be done well, or why anyone should care whether it's done well. I wouldn't understand it anyway. I do you the courtesy of assuming it's important in poetry, as in anything else, to do some good thing well -- and I do you the honor of recognizing that you might well have chosen instead to do some vile thing poorly, as some of of your own fellow-poets have done.
And I, on the other hand, won't burden you with the goals, challenges, and thrill of creating a taxonomy comparable to the Dewey Decimal System.