What’s the rationale behind e-mail clients containing calendaring functionalities?

Many e-mail client programs contain calendar features. Since I rarely ever use any calendaring features I fail to see how e-mail and calendar are related.

To me it feels like calendaring is a functionality on its own and thus should be an individual program rather than being tied into an e-mail client. It doesn't really make more sense to me than i.e. sticking a scientific calculator into Photoshop. Also, no communication software I ever used included a calendar, except for e-mail clients.

So is there any good reason for calendars being integrated with e-mail? How did this arrangement come to be?