When a spice or herb is dried, it still contains the ethereal oils. These oils hold the taste and freshness of the spice or herb. By grinding it, the taste is released and activated.
However; if the spice or herb is stored as a ground powder, these volatile oils will evaporate, and in little time only a pale, weak version of the original spice or herb is left.
Therefore, spices and herbs should be ground just before use. This way the oils will be activated and used when they are at their best. The result is not only more, but also better taste.