Writing down your conclusions makes it possible to reflect on and reason about them. Representing conclusions about one thing will remind you of other things yet unconsidered. Writing down one conclusion may make you realise that an earlier formulation about something else can be improved. There are very good reasons then for documenting the design process, in addition to those usually given of providing a record for management, software maintenance or whatever. However, to be really useful as tools for communication and reasoning, the representations used need to tailored to the context in which they are going to be used.
Different representations have different properties.
First, they capture different aspects of what they are used to describe. For example, representing an organisation in terms of a reporting hierarchy would make explicit different aspects of organisational structure than a sociogram recording who communicates with whom. Let us say that by making some additional assumptions it is possible to infer one representation from the other. Even if this is the case, so that one representation implicitly contains the information in the other, it is still the case that one representation may be most useful for one purpose and the other for another. Second, different representations have different cognitive properties.