The water takes the air spaces out of the gravel.
Try for yourself. Get a pail of gravel and sand and then pour in water and you will see the level drop down. The water makes the sand and gravel lighter by the weight of the water displacement so the particles rearrange themselves and fill any voids once the water drains thru
What do you mean by "filling up the gravel on foundation" I know water is used to improve the compaction of the gravel before pouring concrete or paving. And water is used when concrete cures to prevent cracking and aid in curing.
We used to call that hydraulic compaction. As long as the water is allowed to drain naturally the gravel becomes more compact after this process. It is not as efficient as manual compaction but is acceptable for use under non bearing locations.
To make them clean and perdy?
What is the purpose of adding water?
