An optimum moisture content is essential in composing substrates/growing: they must be neither too dry nor too wet, but precisely in between, so that your plants will be happy in them straight away. The raw materials have to be dry to ensure that they can be properly sieved into the right fractions.
Water is then added in a later production stage, to arrive at the moisture content you have specified. Conditioned transport and good storage are of course important to ensure that the substrates/growing media retain the required moisture content.