Tillage is intrusive and destructive to the soil ecosystem. Tillage destroys aggregates. Tillage stimulates opportunist bacteria to consume the aggregates and the biotic glues in the soil.
Aggregates and biotic glues are really important: 1) house for bacteria,2) improve water infiltration and water holding capacity,3) increase the holding of nutrients. Tillage cuts the fungi network, and stimulates weed production.
Mechanical tiller are killer...remember the soil is alive...treat like your favorite pet...you would not run over your favorite pet with a disk or tillage machine...