A typical user will get involved in a wiki in five stages.
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.
This pattern starts with the creation of a single page, and making an announcement on the home page that people who wish to contribute should visit that page to figure out where their efforts would be best appreciated. The community portal usually comes in two parts. The first is where what's going on is discussed, the second is where the decisions of the community are displayed.
Mediawiki software automatically comes with a Community Portal link as part of their standard links. On Wikipedia, this is where anybody who wishes to contribute can get resources allowing them to have the greatest impact on the wiki.