In WSS, every site is provisioned from a specific site definition. This is true for all top-level sites as well as child sites nested within a site collection. Once a site is provisioned from a particular site defintion, it picks up a dependency on that site definition that remains in effect for the lifetime of the site. A site's dependency on its underlying site defintion can never be removed or changed, and the site definition must be installed and remain functional in the farm for the site to continue working properly.
Consider a scenario in which you create a custom site definition and deploy it within a particular WSS farm. Now imagine that you use this site definition to provision the top-level site within a new site collection. What would happen if you attempted to back up the site collection along with its top-level site by using the STSADM.EXE command-line utility and then restore it in another WSS farm? This would not work properly unless your custom site definition is installed in both farms.