Now, let's gets back to the functional design document. Why is it necessary and what purpose does it serve? Functional design documents help developers, testers, and customers to understand the customizations in detail. Following are key benefits of  functional design documents:

• FDDs help the development team to understand the feature and provide a clear scope and definition of what to develop. Function design documents streamline the development process. The development team working on the feature has a clear understanding and answers of all their functionality related questions to start development. Since this document is approved by the customer, the developers only develop customizations/extensions that are approved.
• FDDs help the testing team to understand the feature under development and to develop a test plan around it.
• FDDs provide the customer with a clear vision and definition of the feature being developed. Also, it helps the entire project team to be able to visualize and see the solution much before it is built.
• FDDs provide the baseline of the training documentation for the application support team and business users.