If you are an author, you need to be prepared at all times.
That means having books and bookmarks with you everywhere you go!
You should have some bookmarks in your bag to give out to those you meet or leave behind with a waitress. (Men can carry business cards in their wallets.) And you should have a copy copies of your book(s) in your vehicle, too. Someone is going to say they want one and if you don't have them handy, you will miss a sale!
Since you should be doing something every day to promote your books, this is one small way to accomplish that task!