It offers calendaring features directly in the Thunderbird User Interface. Further integration features, such as e-mail invites or address book integration are planned for future releases.
But it's just hit version 0.9 (i.e. 90%), as has Sunbird, Mozilla's standalone calendar application:
It doesn't need another client application to run at the cost of a larger download and higher memory requirements. Because of its standalone nature, Sunbird does not offer any kind of tight integration with an e-mail application, such as Mozilla Thunderbird.
Calendaring is indispensable for many, especially in business. That's one of the main reasons why Outlook is so popular, and why open source must be able to offer a solid calendaring client if free software is to be a viable and complete alternative to proprietary programs.