Humidity control is a critical component in the design and operation of data centers, though it can take a backseat to the more obvious issue of heat generation.
Too much humidity, and you risk condensation, internal corrosion, and electrical shorts; not enough humidity, and you risk electrostatic discharge (ESD). Not only that, the rate-of-change of humidity can cause sensitive equipment, such as tape media storage, to expand and contract. This can lead to damage or premature failure.