Software (and systems) exist to do one of a few things:

• provide a service (eg: entertain people, run a bank)
• solve a problem (eg: plan a rocket launch, design an aerial, estimate drug safety)
• augment or replace dull labour (eg: accounting, pin manufacturing)

These purposes come with a range of uncertainty, eg: pin manufacturing is very well defined, whereas entertaining people is very vague.

The traditional approach to software creation (ie: the waterfall method) was created when the majority of purposes were well defined; solving known problems or augmenting labour, and thus assumes low uncertainty. This means requirements are well defined, and software (or systems) can be created efficiently in a production-line-like flow. The efficiency of creation is the highest priority.

Many more recent purposes (indeed most software purposes) are vaguely defined; providing services that can change over time, breaking the primary assumption of the traditional approach, which fails to keep up as there is slow feedback from downstream consumers back through changing requirements and delivery of changes. The Agile Manifesto defines a new approach to creating software (or systems) that focuses on fast feedback and the measures the effectiveness of the resulting software (or system). The effectiveness of the software (or system) is the highest priority.