A DBR will fix some corruption of assets but an EDBR will perform a much deeper scan that involves putting every asset through the same compliance tests that are performed when a new asset is installed. So the EDBR will detect many more errors than a DBR.
My understanding of the repair process is that repairs are easiest to perform on corrupted built-in assets because the original assets are stored in the installation files. So a corrupted built-in asset is simply replaced by its original copy. This is not to say that repairs will only be performed on built-in assets.
Both a DBR and an EDBR will check and add any new assets that have not been installed using Content Manager.