If you want complete protection for your PC then you need to keep your antivirus database up-to-date. This is necessary as on regular basis new viruses are coming into the online world. AV companies keep updating their database so that their antivirus program can fight against all sorts of viruses.
By default, McAfee automatically gets updated but at times it may miss the update. And in case you have disabled the automatic update then too you will face the issue. This is when you can manually update your antivirus software to avoid any sort of virus and malware attack. The update will help avoid other antivirus issues as well.