From the number of emails and calls I get on community each week, it's very clear to me that people have very different ideas when they talk about community. Some talk about blogs when they really mean a group discussion forum, others ask about forums, when they really mean a live chat system, and so on.
Even within a particular service type, such as a forum, there are many models of how teams make use of the service. For example, many teams think of a forum particularly as a product support area. Others thing of it as a way for community members to discuss ideas and new topics. Still others perceive forums as a social gathering/group blog-like atmosphere.
Take another example of a chat system: many have asked us for chats which are more like a presentation with a group of experts that others can submit questions to. Others ask for a free-form open chat room associated with a topic where anyone can ask any question. Still others, consider chats as a private meeting only for a specific group of people.
It's also not limited to a single service either. For example, some want a community service where it's mostly a free-form discussion forum, but occassionaly they can save some information to put into a FAQ. Others want a group document/wiki along with a chat room or forum to discuss some aspects of the document project they are working on. Still others want a blog where occassionally the blogger can have an open chat with people.
My point is that there are many use-cases of these services. Such community use-cases are often repeatable or reusable for different populations or teams. For our site, it's very handy to define such use-cases because the next time you use that model, you have a better understanding of what to expect. Also when people ask for features of the community they want to create, you have a list of use-cases that you may be able to pick from (or create a new one).
From a super-community (a community of communities) like our dW Community, it is even more helpful to have this because you can learn by experience what works and what does not. You can also record best practices on how to interact with the community if you are an outsider, or even within the community.
This kind of semi-formalized approach isn't always perfect or successful but like any kind of knowledge, applying some kind of structure can help in the long-term. This is especially good for the "wild wild west" for new innovative ideas like Web 2.0
Implementing Use-cases of Web 2.0 and community