How will knowing the plant hardiness zone for your area help you plan a garden?

It tells you which plants can usually grow unprotected in your area. If you live in zone 6, and a plant you want to grow is hardy to zone 6, you can grow that plant unprotected all year. If its only hardy to zone 7, you need to protect it for the winter.

In general, if a plant's hardiness zone is less than or equal your garden zone, it needs no protection. If greater, protect it.