First of all, since your data is stored on a secure server in the Yahoo! Network there has to be some way to associate your information with you. Your sign-in enables us to do that. Second, when you sign in to Yahoo! Bookmarks, you ensure that your information is viewed only by you. It's your personal, password-protected collection of bookmarks.
Signing in also lets you retrieve your saved bookmarks from any computer with an Internet connection. You're not limited to using Yahoo! Bookmarks only on the computer where you originally saved the links. Yahoo! Bookmarks is available to you anytime, on any computer you sign in to.