Although developers can and should decompose large problems into a series of smaller ones, at some point they must integrate everything back into a single working whole.
Some experts recommend Continuous Integration (CI), but at the very least we can say that development teams should integrate sooner rather than later.
Integration should include an integrated build of the entire system, and also testing of the integrated whole.
Integration of a system’s components is an example of the “check” step in the Plan-Do-Check-Act cycle, and Continuous Integration (or something close to this extreme) should be considered an example of shortening your feedback loops and making problems visible.
Projects that defer integration until late in their efforts almost always run into problems, and very often those problems are large enough to cause the project’s implementation to be delayed, if not canceled altogether.