This is key for both Field Service and Universal Resource Scheduling.

For Field Service Work Orders, certain Resources are Required on certain Work Orders; for example, because of clearance to access a particular building on a particular military base. Not realistic to create a Skill for each building at each Base. The RSO needs to ensure the Work Order is scheduled only to that or those Required Resources who are able to access that particular location.

For URS, even more important. If I am the Owner of an Opportunity, and the RSO schedules someone else to work on that Opportunity, that's a very serious problem - someone else could wind up working on my Opportunity. Or Case. Or following up on a Lead assigned to me; etc. Need to be able to set the Owner as a Required Resource, and ensure the entity (Case, Lead, Opportunity, etc) is only assigned to the appropriate person.