The template repository manager ensures that the states of the instances in the repository are consistent and up-to-date with your source files when you specify -instances=extern.
For example, if your source files are compiled with the -g option (debugging on), the files you need from the database are also compiled with -g.
In addition, the template repository tracks changes in your compilation. For example, if you have the -DDEBUG flag set to define the name DEBUG, the database tracks this. If you omit this flag on a subsequent compile, the compiler reinstantiates those templates on which this dependency is set.