@tolleranza - Your husband must be a guitar veteran. A humidity measurer and in particular a psychrometer is a must for good guitar care. Here's why: both high and low humidity affects the guitars sound box. Both will make the soundbox arch (one will make it arch in, the other will make it arch out).
And actually, and I believe your husband can back me up on this information, acoustical instruments are actually at their best sound-wise when they are dry to medium dry. Therefore the humidity check is also to make sure the guitar stays relatively dry.