Signing a contract saying you are now one is simplifying something so deep... a relationship is what matters - the customs and ceremonies just complicate things
And you're oversimplifying the impact of this ceremony. Aside from providing a milestone which is witnessed by the friends and family of the couple- it provides them with a supportive community that ideally comes together to help the couple in times of distress.
Most importantly, there are many legal benefits that the couple can only enjoy once the marriage has been completed and recognized. Legal rights such as joint ownership of property, tax benefits, adopting children, and the right to hospital visits and medical decisions as family.