Organisations have, depending on their size, quite different needs with respect to wiki.
Larger organisations are rather hierarchical and act like corporations (see CorporationWiki) and usually start with a project to get some experience with wiki (See ProjectWiki).
Smaller organisations often lack money. They usually do not even have a homepage, at least not an efficient one. So they are happy to start with using wiki as a homepage system, distributing the work-load. Next a private workspace is needed (configured as a SubWiki) to fill internal communication needs. The members of the organisation are registered as authors. They use the individual UploadDirectory for pictures and (to document organisation work) and for documents (concepts, funding, ...). Smaller projects like the preparations of events can also be done in the wiki, so experience grows step by step.