Customer Satisfaction Survey - Get the feedback with name and address for proof of comments. Get feedback that may help you resolve an area of dissatisfaction that your customer has. Its better to know and address these issues and your customer will be grateful for your considerate approach.
Customer Information - Sending out survey based forms allows you to not only interact with your customer, which is always a good thing, but additionally, extract valuable information on going. They may inform you of their intentions for more work they are planning to use you for.