For a lot of people, discovering something new that they didn't know about is part of the enjoyment of reading. If you know exactly what you want, you can order it from Amazon. If you don't know exactly which title, but you are looking for something in a certain genre that you enjoy, or by an author you like, a real-live bookseller can help you find something interesting. This works better for fiction than it would for reference or technical books, but the decline of the large-scale bookstores means that this sort of personal advice will become unavailable to large segments of the population. You can't exactly go look at the table full of new non-fiction or trade paperbacks, pick one up and leaf through the pages, and if you like it, use your 30% off coupon and take it home with you today, if you're sitting in front of your Mac logged in to Amazon.(OK, I know Amazon has this great algorithm for predicting what you'll like based on what you and others have purchased, and you can download and read an e-book immediately, but see my comment about mom's basement, again, and there's this thing about actual books versus having to read it on a gadget.)
Wal-Mart is the cause of the decline and fall of small town commerce. Time after time it's been shown that when Wal-Mart builds a store on the outskirts of a small town, business in that town dries up, the jobs at these stores go away (to be replaced by part-time employment at said Wal-Mart), and the money that people would have spent at locally-owned and operated shops goes out of town. All to save a few cents on light bulbs or pickles. (Even big cities are seeing the same thing.) When the town dries up and blows away because there's no way to make a living there anymore, Wal-Mart closes up shop too (another article), and moves on to conquer the next small town. Much has been written about the aggressive tactics that Wal-Mart uses to exact the lowest prices from its suppliers, many of which have had to move US jobs overseas in order to meet these demands, or have even gone out of business because they could not continue to sell goods to Wal-Mart at a loss.
To turn the subject back to books, Wal-Mart happens to also be one of the largest book and music retailers, and they are known to censor what they sell, to the point of requiring publishers to provide expurgated versions of books and CDs. (Maybe it's a good thing we have Amazon, then.)