Participatory design (and initial design in general) tends to be done at a low degree of
resolution. The typical document consists of hand-drawn screens or mock-ups.

At some stage, the design must be converted to a format that is more formal, and better
suited for use by developers and others. Moving from mock-up to a formal document
encourages the design team to consider proposed solutions in more depth.

Providing a formal specification also minimises the need for developers to second-guess
design decisions.