Writing an application is just the start of a long process. A software system requires a continuous development effort to keep it running and working. Software evolution and maintenance is what keeps busy any organization that creates systems that need to be working for many years.
Software needs to continuously be updated, bugs need to fixed and an organization needs to be able to release new features in a timely manner.
Software Engineering is all about software design, creation and evolution. A lot of different tasks that make a system able to be alive and stand the test of the time.
A nice new book explains how Google manages its vast repository of its source code and its software evaluation as scale. A highly recommended read.