For the writer: It does everything a business card does, but better.
1) When you, the author, meet someone at random — whether at a booksigning, on a plane, in a store, or socially: “Oh, what do you write?” You want to be able to offer a leave-behind — something to reinforce to that person that they were interested in you, and look, here is your book. A business card doesn’t contain info about your latest book release (unless you are updating your business card that often, in which case, um, yeah — we want your business, for sure!)
2) A business card contains your personal information. And really, besides your website address, do you really want to give this random person any more info?
3) Handing a business card in this instance is inappropriate. This person is not a colleague — he/she is a potential reader. (Unless this person is a colleague, in which case you need a business card, too.)
We have lots of reasons bookmarks are good business. Stay tuned.
(And yes, we do design bookmarks. Thanks.)