Both exported and virtual resources are declared once, and are then collected in different contexts. The syntax is very similar, as are the concepts.
Sometimes, a central definition of a resource cannot be safely realized on all of your nodes, though; for example, consider the set of all your user resources. You will most likely wish to manage the user ID that is assigned to each account in order to make them consistent across your networks.
This is often solved through LDAP or similar directories, but that is not possible for some sites.
Even if all accounts on almost all machines will be able to use their designated ID, there are likely to be some exceptions. On a few older machines, some IDs are probably being used for other purposes already, which cannot be changed easily. On such machines, creating users with these IDs will fail.
The...