What's the purpose of the salesforce communities licenses?

The Salesforce communities licenses - customer community plus & partner community are nearly identical except that in addition, Partner Communities licences have read access to Campaigns, and full Read/Write to Leads, Opportunities, and Events.

Partner Community Licenses were created to allow collaboration with partners, to allow you to go to market together, hence the access to Leads and Opportunities. Customer Communities licenses allow you to collaborate with customers, on things like support cases and account administration. Customer Communities Plus is aimed in the middle, at more robust customer collaboration and exposure of many more Salesforce objects and the sharing model.

A full licence matrix put together by the fine folks at TQuila can be found here: https://docs.google.com/a/salesforce.com/spreadsheet/ccc?key=0AuSsu_7yn04IdDRxajlZM196aGM1bUx2MHlMdERraHc&usp=sharing#gid=0