The control center is an optional feature that is required only if you are unable to set up the desired options through the default "display settings" interface on Windows.
In the past, I have used the control center to configure the relative positioning of monitors, but the native support for managing monitor positions have improved (it is fully usable in XP) in Windows, thus making the software entirely unnecessary for my use.
Some of the features you will or will not miss are described on the official ATI page and includes functionality such as:
- wizards for managing monitors and 3D settings
- monitor positioning
- fine-tuning of video setting (hdtv, codec parameters, colors, brightness)
- fine-tuning of 3D setting (system wide, per application)
For the most part, one can do without these features, and the control center is not required for the driver to work properly.