Hi Islam,
from the KB article:
As part of SharePoint database upgrade, the members of the db_owner role are deleted from the role, except for the “dbo” user and the user who is performing the upgrade. This is done for enhanced security to make sure that an errant account isn’t inadvertently left having the db_owner role privileges. However, there may be situations in which this behavior is not desirable. For example, RBS may be broken every time that an upgrade is run. In this situation, a registry key is added to bypass this behavior.
With other words: this is not mandatory – this is optional and only applies to certain specific situations.
Cheers,
Stefan