What is a reveal cleat (and why do I need it)?
It appears to me that the reveal cleat serves two functions. 


It keeps the oven set in the middle of the opening  so that the reveal on the face frame of the cabinet is equal on both sides.

The reveal is the amount of the narrow strip of face frame that is visible between the trim of the oven and the edge of the face frame opposite it. 
It maintains an equal space between the side of the oven and the cabinet sides. ( inside the space of the cabinet )