Repositories are a new concept for XF2, but you might not be blamed for comparing them to the "Model" objects from XF1. We don't have a model object in XF2 because we have much better places and ways to fetch and write data to the database. So, rather than having a massive class which contains all of the queries your add-on needs, and all of the various different ways to manipulate those queries, we have the finder which adds a lot more flexibility.