You and the customer sign a contract, it is filled with rules and conditions, loopholes and fine prints. The goal is clear - do something now so to deliver as little as possible in the future.
This is about the past.
In a handshake engagement, either party can claim loopholes and has no commitment. The consequence is clear - if you disappoint us in the future, there will be no more handshakes.
This is about the future.