I think it's safe to say that publishers have come to the conclusion that "pretty covers" attract more attention than interesting titles... witness, the predominance of cover art that has taken over the publishing zeitgeist over the past century (you used to just get titles and authors on a cover, now it's almost unheard-of to get only that). The recent trend of putting thumbnails of covers on book spines is an extension of that idea.
Being able to tie visual cues to memories is proven to be a more effective way to remember items. So it makes sense to have covers and to display them, in order to make it easier for readers to find and remember books. I'm sure it is easier for browsers to find a book that interests them, or a particular book they are looking for, if they can scan visual images instead of lines of text.
Many of the reader apps I have on my smartphone display covers when looking through my collection of books... not just when I open the individual files. I think it's a bit more attractive a way to present books, rather than simple lists, and makes it easier for me to find what I'm looking for.