OK, so what is a green manure? A green manure is a crop planted to keep soil covered and add green and root mass to the soil for additional organic matter.
Naked soil blows away with wind and washes away from rain. It also is susceptible to wide temperature swings that harm or kill microbial life in the soil. A crop (or mulch) acts like an insulating layer to temper soil temperature swings.
Organic matter in the soil is key to many elements. Organic matter lightens heavy soil and adds mass to light soils. It creates a sponge - like texture that means soils are aerated and when rain falls, there is space to absorb water like a sponge, so that soil with lots of organic matter can slowly release water so uneven rainfall is less of a problem for crops.
Ken read that for each percent of organic matter, crops can use the equivalent on one inch of water per week.