It's not for finality, it's for authentication: so you can prove later who is responsible. A private stamp or seal has been used historically for the same purpose. A signature has the major advantage that you can use an ordinary pen - or, when they were first invented, clay marking tool. The oldest signature which survives today is from the scribe Gar Ama signing his work 5,000 years ago. Seals, also used originally on clay, are even older.

Another AskHistorians question which might interest you: http://www.reddit.com/r/AskHistorians/comments/1dpxl4/how_secure_were_wax_seals/