Reserve at completion - best practices

What is the significance of ‘Reserve at completion’ flag available under Purchasing > Setup > Organization > Financial options > Encumbrance tab ?

If “Reserve at Completion” is checked, the functionality allows the users/preparer to take Reserve action on the Requisition before it is submitted for approval . In this case the ‘Reserve’ checkbox is enabled in the approval window for Requisition.

If “Reserve at Completion” is unchecked, the functionality prevents users/preparer from taking Reserve action on the Requisition before it is submitted for approval. This keeps the system from reducing the Budget (by reserving a Requistion) before it knows whether it is going to be approved or not.