When designing the data warehouse for a company or organization, we have to make sure that it will be a value-added system. Also, we have to make possibilities for the data warehouse to be developed in the future. That's why we need SDLC methodology for data warehouse design.
Here are nice quotes about the topic.

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"Accurate requirements definition is many times more important for a datawarehouse. Because the datawarehouse environment is an information delivery system where the users themselves will access the datawarehouse repository and create their own outputs." (Datawarehousing Fundamentals, Paulraj Ponniah, John Wiley & sons.)

The Systems Development Life Cycle (SDLC) is the process of understanding how an information system (IS) can support business needs, designing the system, building it, and delivering it to users.
(System Analysis & Design, Alan Dennis & Barbara H. Wixom, John Wiley & sons)
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