Why is the angle of the horizontal stabilizer marked on the fuselage? Why are the angles displayed on the fuselage?

I understand having markings with depth on the hull of boats, but I don't understand the use case on a plane.


Photo of the root of the trimmable horizontal stabilizer on an Embraer ERJ-170 (wikimedia.org).
I don't have the reference but it's showing the trim setting.  The picture looks like is an Embraer 170 on which the entire horizontal stabilizer moves.  The gauge gives a quick visual setting to confirm the the trim set in the cockpit matches the actual trim.