Wooden stringed instruments are built light enough to allow string vibration to move the wood and create a sound wave/tone and braced or carved in a fashion to prevent overall string tension from quickly deforming and destroying the sound plate. Examples are seen in flat top guitars, when the bridge twists and no longer sits flat/horizontal to the top and strings, but shoves the front of the bridge (toward sound hole) down and pulls the back (toward the end block) skyward. Taking top and X braces underneath for its ride to near vertical final tweak with no glue joint failure. Carved mandolin tops (under extreme pressure) show signs of stress around the F holes as the top sags under the bridge pressure or as the top bulges between end block and bridge and/or neck block and bridge.
Wood is prone to warpage when enough time and pressure are applied. No matter how dense it may be. Even ebony mando bridges concave under string pressure with a little time. There is no one thing that can make a fine wooden string instrument sound good but, there is one thing that can make them last a long time. Carbon graphite 4 times stiffer than steel by weight. It will not rust in neck reinforcement application. It will stabilize sound plates in flat top instruments. For a carved arched top instrument a combination of proper arching added to the graphite spruce laminate tone bar, spring loaded to push up under the bridge and pull down on the ends against the bulge. It will boost string response levels and in the long run continually provide the strength that will never fatigue with time and pressure. It will combat overall long term effects string tension has on wooden stringed instruments.
There's a reason they pre stress the rebar when building an aircraft carrier. It's so upon pouring the cement for the floor it will settle flat under the weight. So in designing anything that spans a distance under pressure and weight , be it bridge, aircraft carrier or mandolin. Pre stressing against weight and pressure is a logical, beneficial and proven construction technique.
Instrument builders who have experimented with minimum tolerances for a given instrument design usually find a point where the instruments sound really good brand new, but self destruct within a few years or so because of the ultra light construction.
There is a certain amount of stiffness an instrument needs to last for years and produce a stable, full/fat single note (tone/sound). Real light tops produce woof/open bass response and thin treble notes.
Real heavy tops produce thin light bass response and cutting treble notes.
I am generalizing here because wood density levels, bracing patterns, the back and neck all contribute to towards tone production.
The use of graphite in the tone bars allows the builder to confidently remove wood to bring out the sound and still keep the stiffness needed for tone and longevity.
Some call it a joke some call it genius, but its just another material at the luthiers command to help take his instruments further into the future. To bless the following generations with that magical thing called music.