Perhaps most importantly, a good garden should feel safe. The real test is whether you'd feel ok taking a nap there. Historically shelter may have kept us safe from animals. But today shelter is still relevant. Now we take shelter from modern distractions.
Shelter also comes in the form of the what plants are in the garden, ie. not poisonous ones. Stone walls serve as physical barriers and reduce sounds.
Check out posts that show other examples of how gardens shelter us.