The reason for introducing that rather challenging mental gymnastic exercise is that water vapour in air is an important influence on museum objects, many of which contain water in a loosely bound form.
Wood is typical of the water-containing materials.
Wood in furniture contains roughly ten percent water by weight. The exact water content depends on the temperature and humidity of the surrounding air during the previous few weeks. The water content matters to conservators because wood swells and shrinks at it absorbs and desorbs water. Water is also an accelerator of slow decomposition reactions. If we take a five liter jar we can calculate, by reference to the Mollier diagram and the density of air, that the air within it, at room temperature, cannot contain more than about a tenth of a gram of water.