Once you start using some desktop features such as sticky notes, it becomes difficult to revert to not using them. OK, there not a massively important application, but they are useful. The same goes for a simple desktop calendar, because they are so easy to refer to. More heavily featured desktop calendars will enable you to theme or change the way they look so you can incorporate it with your desktop, amongst other useful features.
Active Desktop Calendar 7.50 will show appointments and tasks on the desktop, enabling you to colour code, change fonts and mark dates with notes and alarms. You can also organise your data and share it with people on your local area network, without showing any notes that may be private.