In sync client builds below 18.171.0823.0001, this setting only redirects empty known folders to OneDrive (or known folders already redirected to a different OneDrive account). In later builds, it redirects known folders that contain content and moves the content to OneDrive.
I recommend using this setting together with "Prompt users to move Windows known folders to OneDrive." If moving the known folders silently does not succeed, users will be prompted to correct the error and continue. If you enable this setting and provide your tenant ID, you can choose whether to display a notification to users after their folders have been redirected. If you disable or do not configure this setting, your users' known folders will not be silently redirected and/or moved to OneDrive.