Cello humidifiers are normally inserted into cello cases to assist with the regulation of moisture in the air (or within the case). As wood interacts with moisture constantly, if the air inside a cello case is too dry, it may suck out the moisture from the wood of the cello altering the instrument's playing capabilities. The opposite may happen when too much moisture is present and the cello wood may absorb the excess moisture, causing swelling. When a cello is exposed to extreme variations for long periods of time, open seams or even cracking may result.
A good humidifier will protect the instrument in cases of both high and low moisture in the air and is a "must" especially in areas where humidity varies greatly.