What is the mechanical reason for rolling the same initiative for a group of creatures?


The 5th ed Players handbook states that

The GM makes one roll for an entire group of identical creatures, so each member of the group acts at the same time.

Historically in other systems I have rolled initiative for all NPC’s individually, allowing for the fact some may react quicker then there counterparts.

Has the reason for this bulk rolling of NPCs ever been explained, or is it simply a mechanic to make things run smoother?