This is carried out in Autumn to expose large clods to repeated freezing and thawing. This has a very real effect, it produces what old gardeners called a frost mould: beautiful crumbly soil. To benefit from this effect in Spring, it is important not to turn over the soil again. Get a seedbed by shallowly working through with a rake or fork.
What non-diggers say about this
1.) The effect is short term and anything that creates loose soil exposes crumbs to slaking down in heavy rain, exposing the soil particles. It is important to understand that soil crumbs are just loose associations of particles and are notoriously unstable. Never produce excessively fine seedbeds, they tend to ‘cap’ after rain and bake hard in the sun.
2.) After a year or two, although non-dug soil is firm and settled, accumulated worm casts make it easy to create a seedbed.
3.) Non-dug soil still gets some benefit from the cold.
4.) Non-diggers do not like leaving bare soil open to the leaching of nutrients. We would rather grow plants!
Important technical note: before we children throw our toys out of the cot arguing, let us be clear of one thing. Most soils are mixtures of sand, silt and clay particles. Nothing the gardener does changes the fundamental nature of these particles.
This gentleman strongly disapproves of non-diggers.
He is chairmen of the diggers fan club!