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.