One of features which makes any email client (either an application installed on your computer or a web product) more complete is a calendar. Email and calendar applications go hand-in-hand as the actions or events in one commonly lead to an action in the other. If Thunderbird is your email client of choice and you want add the calendar functionality, take a look at the Mozilla Lightning project.

From their web page:

Lightning brings the Sunbird calendar to the popular email client, Mozilla Thunderbird. Since it’s an extension, Lightning is tightly integrated with Thunderbird, allowing it to easily perform email-related calendaring tasks.

Nothing revolutionary here, nor does it need to be. The benefit here is your email client of choice becomes more ‘complete’ with the addition of calendar functionality.