It all depends on if you do this through "Windows Backup" or directly with "System Image". Windows Backup allows you to also create a system image with it if you like. Otherwise, you could just use System Image to create only a system image without Windows Backup for folders.
An example for why would be say someone wants to manually create a system image only once a week, but would like to keep important files in a folder scheduled to be backed up everyday (or however often).
Some businesses or folks may also wish to use Windows Backup and also have the system image option checked to have it all created automatically on a schedule.
It just depends on what their needs are, but it's great to have options.