When you need to take responsibility for a service during its full life cycle:
Clearly recognize two distinct phases in the life-cycle of a service: implementation of a number of changes (also known as development or design), and the service being in operation. Also recognize an intermediate state: deployment, a transitional phase when the service is put into operation.
Service design (or development), service transition (or deployment), and service operation, have distinct needs. For service design, it is important to have a solid understanding of the problem domain and what solutions that meet the needs within that domain. For service operation, it is important to have stability so that people can rely on the service being there.
Since the best understanding of needs occurs when a person actually use a service, the chances of meeting them increase when we can develop features and put them into operation continuously. For that we need to design a deployment process that is fast, stable, and predictable.
Note that these three processes are run in parallel and in a flow. Needs are met when they are quickly identified, and solutions are quickly being designed to meet them, and the solution is quickly brought into flawless operation. We need to optimize the correctness and speed of this flow, while understanding that development, deployment, and operation are three distinct phases with different requirements.
This is basically the notion of "Service Design", "Service Transition", and "Service Operation" from ITIL.
(ALMA: Agile Lifecycle Management Anatomy)