Does the coop need a door that closes to keep the Chicken's inside of their house?

I'm totally new to raising Chickens. We started yesterday with 3 ISA Browns chicks. I purchased this coop today but it doesn't have a door that closes to keep the chickens in the coop. I see that a lot of coops do have doors that people must close.

You can see where they walk up the ramp to get into the coop, but there isn't a door over that (approx.) 12" x 12" opening.

What is the purpose of the door and is it needed or not?

We live in Michigan where winter nights get to 10 degrees.

http://common2.csnimages.com/lf/49/...ision-Pet-Products-Hen-House-Chicken-Coop.jpg

Thank you for your help!