Good air circulation is important for a healthy garden. When air flow is limited, the garden can remain damp for long periods. Dampness is the perfect condition for many fungal infections like powdery mildew, as well as a big draw for many pests, such as slugs. Air needs to be able to flow through the plants.
If the leaves of your plants are not swaying with a slight breeze, your garden is not getting good air circulation.