Moisture or water vapour is one of the components which can cause an undesired chemical reaction.
For instance, Lithium (used in the production of Lithium batteries) will react violently with water vapour. This is why production of Lithium batteries can only be done in an extremely dry (dehumidified) area.
Also explosives and propellants are known to be sensitive to chemical reactions with moisture. Undesired reactions might lead to an unstable product (danger of explosion) or the forming of unwanted and dangerous chemical compounds (for instance hydrogen gas)......