Manure only does not help much.
Proper tilling of the soil, creates air gaps which trap the moisture, and hence prevent the evaporation of water.
When manure is added, it increases the fertility of the soil. Thus more microorganisms and earthworms live and thrive in the soil.
When the microorganisms respirate it makes the soil more airy.
When the earthworms move around eating all their food and excreting precious nutrients into the soil, they not only make the soil fertile, but also till the soil and create smaller air gaps.
These occur all through the day.
Thus manure assists in the formation of soil gaps.