I am no Soil Scientist but this is the way that I think of it ...
Soil is made up of several components, including ...
Mineral : Inert material from rocks. Might be sand, clay, gravel, whatever
Humus : Organic material of plant origin. Including that from manure.
Micro fauna: bacteria, microbes, etc which feed on humus
Worm casts : some of each of the above which has been 'processed' by the worm's gut
[/list] As such, worm casts are simply superb for the gardener. The nutrients are in a form highly available to plants, the crumb size is perfect for root hairs. There is nothing better in my book.