Because software is never finished...
Part of the everyday experience of countless IT Project Teams and the companies that serve them is that the functional requirements of a software solution may undergo significant mutation, not only after implementation, but also during development.
This means that if business applications are going to bring durable and genuine value to a client, they will need to demonstrate a high degree of flexibility in favour of modification and evolution.
Experience teaches that traditional procedures such as V Model, RUP (Rational Unified Process) and classic phase models fail to match the daily realities of software development. These processes, like the solutions that are built using them, are simply too unwielding and sluggish to adapt to the kind of changes that occur in modern-day development and implementation.