Many, if not most, Android devices will be phones. As such, not only will users be expecting to place and receive calls using Android, but you will have the opportunity to help them place calls, if you wish.
Why might you want to?
Maybe you are writing an Android interface to a sales management application (a la
Salesforce.com) and you want to offer users the ability to call prospects with a single button tap, and without them having to keep those contacts both in your application and in the phone's contacts application.
Maybe you are writing a social networking application, and the roster of phone numbers that you can access shifts constantly, so rather than try to sync the social network contacts with the phone's ...