Contracts and extensions provide ways for an application to integrate better with Windows and other applications; for example, users can share data by using the Share charm, regardless of the kind of app. Contracts and extensions provide consistency in the user experience, not just the programming model. This makes the application more useful; it looks as though a lot of thought and care has been poured into the app. In general, this makes the app more likely to be used—which is one very important goal when building applications.
In the next (and last) chapter we'll take a quick look at application deployment and the Windows Store.