||The mirror allows you to view the compass dial and the background
at the same time.
Notice the vertical line in the center of the mirror. This vertical
line can be extended (conceptually) into the horizon to more accurately
target your desired bearing. And the fact the the compass dial can
be seen at the same time the target is aligned, makes mirror compasses
better for taking accurate bearings.
Adversely, with a baseplate compass, you can not view the compass
dial and the target at the same time.