You just don't want to have it touching the trunk of the tree, there's no magic number to keep away.
It does apply to all long-lived perennials, especially those with woody trunks. There are plants, such as tomato plants that this does not apply.
The reason for this, is that the bark of plants are akin to the skin of us humans, i.e. first line of defense of the immune system. If you pile mulch against the base of a tree, it traps moisture and allows things to grow, i.e. fungi/bacteria and as time goes on it breaks down the bark and gets inside the plant and also allows other things, such as ants to enter... This is also why you don't want to bury a tree too deep when planting/trans-planting -- a significant cause of tree failure is because they were planted too deep.
An analogy would be for us to wear boots 24/7, eventually your skin would breakdown and you'd become sick.