A deep, organically rich soil absorbs more nutrients and water and your vegetables will be more nutritious and flavourful. Good healthy earth should be dark and easy to break into clumps.
If your soil is silty, sandy or compacted, you may need to aerate and enrich it. Turn the soil over with a spade, add fertiliser, then topsoil and allow this to sit for a few days before you plant.
Going to pot? Use potting rather than garden soil and top with a layer of mulch to retain moisture. Containers, particularly clay pots, dry out faster than ground soil.