Then you can use google also.
On-Access/Resident scanners scan activity, when access is made to a file (creation, modified, opened or executed) - Most antiviruses are on-access/resident and actively monitoring and scanning your activity all of the time monitoring and scanning your system.
On-Demand Scans - Means that you the user manually run them (or set up a scheduled scan).
So running these on-demand scans when you have on-access scans being done by the installed antivirus, means your active files are/have already being scanned (duplication).
So what other files are going to be scanned by the on-demand scan, effectively the ones that aren't active/dormant. If for any reason they become active, then the on-access scanner/s would scan them.
This is where my comment comes from - With a resident on-access antivirus like avast, the need for frequent on-demand scans is much depreciated.