Dom, if you enter your approximate location (city and state) into your personal profile, ATP will automatically tell you which zone you are in, I think. Click on the little icon of a person in the blue sidebar at left of the home page. Then click 'Change your public profile'.
Knowing your zone allows you to choose plants that will survive winter where you live. Or in my case it also helps me to choose plants that can survive our long, hot humid summers. But if you only grow plants indoors, it's sort of irrelevant.
The NE part of the US has zones ranging from 3 (very cold in winter) to 6 or 7 near the coasts, which have their winter temperatures moderated by proximity to the water. Seattle WA and Vancouver, Canada also are in zone 8 because it rarely freezes there for long, the sea water moderates the temperature so much.
Altitude also affects what zone you are in. My daughter lives in Salt Lake City, and is in zone 6 but just a few miles away, up in the mountains would be zone 3 at best.
