Partner Community sites are a forum to connect with a targeted subset of users that may be internal or external to your organization.
For TaroWorks, we use communities as a way to communicate with mobile users in order to reduce licensing costs to our customers. (Full Salesforce licenses are more costly than Partner Community licenses from Salesforce.) When your TaroWorks mobile users have internet access, you can also use your community site to share additional information with them outside of TaroWorks such as reports and dashboards. Another example is that some of our customers are planning to use community sites to share information with donors, which are external to their organization.
For more information, please visit salesforce.com.