The difference comes when you want more in depth detailing like Reporting security configuration and access providing and information visibility.
For example consider as part of Ob-Boarding process you have a process for assigning laptops to the users. This assigning laptops to the users will be done by IT Department. If we use the approval and providing the things with in the same flow then when the IT Department wants the list of laptops that are newly assigned every month they cannot get information as this is On-Boarding case where few people requires laptop and few not.
Now the same scenario if we go we a separate child case where they provide all the details of allotment every month end they can have the statistics of how many allotments they are doing and they can do predictions using reporting.
In addition to the above same applies to Security configuration also.