Calendar based features for your apps
The apps with calendar or scheduling based features attached start to be aware of the schedule of their human user. You can use that to personalize the content shown to your users to enhance the user experience. On the other end you may need just a blackbox to store and manipulate calendar data. Something you just use and do not have to care much. 3e Calendar will be the right tool for you in both cases.
Develop or integrate faster and cheaper
Easy Event Exchange protocol is based on on XML-RPC over HTTP. These are the technologies available in any development environment without any extra special APIs, libraries or expertise required. With EEE you will need less effort to develop your calendar aware apps.