Guest books also seem to be commonly used before or after a big event.
Sports events, for example, might use an online guest book for people to leave messages encouraging their team.
Or after the death of a public figure or a major disaster, guest books are often made available for people to write messages of sympathy or encouragement. It's a nice gesture, particularly for people who can't afford to offer financial assistance during these events.
I think, more often than not, the act of writing the message is even more important than if anyone ever reads it.