Humidity for wood rotting
Rotting is a biochemical reaction: basically catabolism (breaking down of complex compounds to simpler compounds). Cellulose or lignin or any other structural polysaccharide will not undergo spontaneous hydrolysis. This reaction is catalyzed by enzymes produced by the microorganisms like fungi and bacteria (the term for these decomposers is saprophyte). Saprophytes prefer to grow in humid and warm environments.

So the primary role of humidity is to promote the growth of decomposers. Simultaneously it provides the water required for the hydrolysis.