You're welcome. The Mark/Space documentation does state how important it is to overwrite the contents of your handheld during the first synchronization event, but it doesn't explain why.
The reasons are rather complex, but in relatively simple terms this step is required for each data type because of requirements imposed—and inherent limitations of—the Sync Services framework. A great deal takes place in the background when you introduce new Sync Services-savvy conduits and applications, including requirements that these new clients [devices, applications or servers] be registered with Sync Services and their schemas recognized. Others may have to be unregistered at the same time. And, there are inherent limitations in the framework: for example, it has no provision to allow your handheld to overwrite either iCal or the Address Book. The standard Palm conduits do provide that option, but they continue to work independently of the Sync Services framework.
If you are interested in what's going on behind the scenes, here is a link to an Apple Developer document that explains the basic structure of the framework:
http://developer.apple.com/macosx/syncservices.html