Change is a fundamental element of consciousness. It is what calls our attention, awakens it, stimulates our questions. We see the red coat in contrast to the white snow... Without change, our minds become dull and unaware.
I believe that angels are forms, images, and expressions through which the essences and energy forces of God can be transmitted and that, since there are an infinite number of these forms, the greatest service anyone can pay the angelic host is never consciously to limit the ways angels might appear to us.