WSUS post installation tasks is mandatory and this task will create the database (SUSDB), directories, pool in IIS,import the publisher info such as windows,office,SQL etc and other important tasks for ConfigMgr to recognize that, WSUS is ready to be used for synchronization with Microsoft.
If WSUS is not configured after its role installation, you cannot ConfigMgr for any patching activities because the SUP role will fail to install because it has dependencies on WSUS configuration.
Regards,
Eswar
www.eskonr.com