A typical user will get involved in a wiki in five stages.
Reading the content Making edits as they see them Looking for edits that need to be made Looking for sections that need to be expanded Organizing the overall effort
This is similar to the 90-9-1 rule. When people hit #3, it is beneficial if there is a central place on the wiki where they can join in on discussions about what areas of the wiki could use the most work, and what efforts would be most appreciated. The community portal provides that place.