The Default policy is Always ON and applicable to to Everyone or complete network. Hence scheduling is not applicable here.
Group Policy is a custom rule that can be customized as per your requirement and can override Default Policy if needed. This can be applied on selected devices. You may apply this to all the devices too.
Scheduling option in Group Policy gives you a method of Switching OFF the applied Group Policy for certain time and reverting to Default Policy.
How ever Group Policy will have similar options as of Default Policy.