Couldn't make sense of your post until I had a brainwave and that the title and post may have been edited by the site admin - Where you actually asking as to why use a building material known as the following two words combined together in the reverse order than I have given here "core" and "hard"
If it is then I would have though on a DIY site they knew that this world is the correct term for a material used as a concrete floor sub-base!!!!
Anyway - the reason why you need to use it instead of gravel is that you need to compact it to give a firm base - thus you need the pieces of all sizes inculding the fines (dust) to bond together when compacted - with just gravel you won't get this bonding.
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