I wish all helis were routinely fitted with rear view mirrors.
RAF Puma HC1s had them as an interim mod before Polyvalent intakes. We used them to look for ice, the intended purpose, but as others have said, invaluable for monitoring the condition of the engines - checking fire warnings, oil leaks, panel security etc. Not to mention the tail rotor and who is in your 6 o'clock!
We were horrified to see the engineers removing them a part of the 'PIP' intake mod. scheme. We asked to retain them but were told it wasn't possible!