To grossly simplify things:
MRP is, basically, for matching supply to demand. So if a job is going to be late for an order, it’s going to suggest you do something. Otherwise, it doesn’t necessarily care. Finite capacity just means it’s going to try to plug the unfirm job into an open slot.
Global scheduling takes the entirety of your jobs, ranks them by priority, require date, and the time needed for inputs to arrive. If possible, it will backwards schedule everything from the due date and be done. However, if you’re constrained it makes tradeoffs. It’s not really looking at demand… It will not create new jobs to fill unmet demand. It will not tell you your require date is out of line with what you really need. That’s what MRP is for.
So you need both. MRP to satisfy demand with new jobs and PO’s. GS to prioritize the jobs you’ve got already. MRP is a crappy scheduler. GS will never create a job and almost blindly follows the require date on your job as its goal.