On Friday I talked about book buying and a purchasing-eye-view of what's up in publishing, in which I emphasized the importance of immediate gratification in our cultural buying habits. If the book's not on the shelf, you risk the reader picking up something else instead.
Now for the other side of that coin: Every book is a tiny monopoly.
This is the same in the gaming industry as it is in books. Someone who goes to the store to purchase "a game" or "a book" is likely to leave with "a game" or "a book" regardless of whether they had a specific item in mind. But people who are looking for a specific book or game that is out of stock are much more likely to leave without buying anything.
Bob goes to his local game store to buy a game because he's having some friends over on Friday. He was sorta thinking about getting Uno, but the store doesn't have it, so he gets a copy of Fluxx instead. He might also pick up another game or two, just in case the first one sucks, or maybe a few packs of Magic cards, or some shiny thing that catches his eye that he didn't know was available. (We gamers like shiny things.)
The Settlers of Catan is one of the most successful board games of all time. If Bob goes to his local gaming store specifically to buy Settlers and the store has it in stock, Bob is going to buy that copy of Settlers, and maybe an expansion for the game or something shiny as well (see above). Whereas if the store is out of Settlers, then Bob is most likely going to leave and try somewhere else without buying anything.
Why? Because he doesn't just want "a" game. He wants "that" game. Settlers is its own tiny monopoly.
The same is true for books. If Bill goes to Barnes & Noble just to browse, he's probably leaving with at least one book even if the specific book he was kinda thinking of buying is out of stock. But if Bill goes to Barnes & Noble looking for a copy of Chalice by Robin McKinley, and the store doesn't have it, it's off to the next store, or off to order it online. (If you have to order it, you might as well order it online where it's going to be cheaper, arrive faster, and probably won't cost you shipping if you combine it with those impulse buys you would've made anyway.)
What all this boils down to is that, as an author, I need my book to be on the shelf. If it's not on the shelf, that big distribution channel is kinda useless. If my book isn't big enough to warrant a chunk of that shrinking shelf space, then I'm probably not going to lose many sales by not being in Barnes & Noble at all versus being in as a special order item.