Why does having two doors to get into your house instead of one helpful to keep heat and/or A.C. in

Because using two doors reduces air circulation exchange from inside to outside as well as heat loss.

Using two doors makes people open the door twice.
When people open the first door, the second door is still close. So there is almost no air exchange from inside to outside, and vice versa.

When people open the second door, first door has been closed, so there is almost no air exchange again from inside to outside