Authors could sit in the storeroom and autograph a hundred books for the bookstore to put on display. No, signings are fun because the author is sitting at a table right by the front door of the store. Friends come in to chat and drink coffee. People who might have read the author's work for years come in to see what he or she looks like in person. Relatives of the author hang about, making the crowd seem bigger.
Book signings are social events in which everyone ought to come away with something.
The bookstore manager, for example, quietly lurks around the fringes of the group, keeping an eye on how many books are being sold. His bottom line receives a nice boost from a successful signing. The author, of course, also wants to sell books but wants some other things even more. He has probably worked for years on this one book, for example, and wants to bask in whatever meager glory might result. He wants to make a good impression on the store manager so he'll get invited back for another signing when he writes another book.