Reason 1; not to bring buried weed seed to the surface
These leeks in my vegetable garden are sown broadcast. Normally gardeners sow in rows so that they can distinguish plants from weeds. ‘One year’s seeding is seven year’s weeding’. According to this old adage, if you kill germinated weeds before they seed, after a few years the soil’s bank of dormant weed seeds near the surface becomes exhausted. Those who dig are always bringing new weed seeds to the surface and are never in control. Of course we all fail to completely achieve this virtuous cycle and wind-born weed seeds still arrive!