The dimensional changes due to drastic shifts in moisture content of your guitar can result in shrinking, expanding, warping, checking and splitting. Dryness can actually pull glued joints of furniture apart. One factor that isn't covered in the engineering manuals is: what does this do to the tone of a guitar? Just the difference in weight of the wood will change the resonant frequency noticeably.

Expansion of wood due to moisture varies with the direction of the grain. Imagine the stress on a piece of wood that is glued to a stiffener that has the grain running in a completely different direction.

Painting or other forms of moisture proofing wood will protect it from moisture. This has been done with the outside of your guitar but the inside is bare, untreated wood.

Your guitar will be happiest when it lives in the same humidity conditions as it experienced when it was being built. Most luthiers seem to keep their workshops in the range of 50% RH.

Extreme dryness can also cause problems because of static charges when you are recording or amplifying your guitar. Static build-up occurs easily when the air is dry. Everything wants to act as a capacitor and store up a static charge. The result may be an excess of popping noises, sparks when you touch the microphone or other objects, static build-up on recording tape, etc. Many of these noises will actually be recorded. Adequate humidity allows static charges to bleed off before they can cause problems. I have solved stray voltage (static) problems in a computer lab by increasing humidity and pouring a few buckets of water on the earth surrounding the grounding rod for the electrical system... [wow! radical move - personally, I'd be careful where you chuck those buckets when dealing with electricity! - Ed]

More technical explanations of relative humidity, as it affects your guitar, can be found here:
http://members.aol.com/_ht_a/Rogluthier/humidity.html