When you let me know that you are looking for a specific author, or a specific book, that tells me what you want that I don't have, and it enables me to get what you want. If there is a problem with a website, or you have a question, and you don't let me know, then I can't resolve the issue-case in point, when I moved to this host from my previous host, there was an issue with the payment screen. I wouldn't have known there was an issue had I not been contacted by someone trying to place an order and getting an error message on that screen.
Taking a few minutes to let me know what you like (or don't like) helps me to make sure I am staying abreast of what you, my customer, wants or needs. Without that crucial information, I can't be certain that the books I am carrying are what my customers want-they are all great books, but if it isn't what you're looking for then they aren't the right great books. I can be contacted through the website, Facebook, or Twitter if you have a suggestion, a request, or just a "great job!" (those are nice ;) ).