A closet dehumidifier is designed to absorb moisture from enclosed spaces, which in turn protects clothing and other valuables from the effects of mold and mildew. Closets being what they are rarely have any form of ventilation or airflow through them. Even though you may think you there is no damp problem, it only takes a bed sheet or garments that are not quite dry to cause problems with mustiness from the stagnant air.
water vapor
A home desiccant dehumidifier makes use of certain materials in order to remove moisture from the surrounding air. This is accomplished by exposing a desiccant type of material to an air stream that has a high relative humidity level. The desiccant material attracts and retains some water vapor and the desiccant is then exposed to an air stream that has lower relative humidity causing the retained moisture to be drawn out of the material.