cats

Sure, you've entered through the first door, but what about second door?

It insulates better than your normal single door, the screen door is self closing so it helps to keep bugs out if you're doing anything around the house where you're going in and out a bunch, and when the weather is nice it's quite enjoyable to leave the main door open so all the light and a breeze and sounds get through the screen door. It's like a window that goes all the way to the floor and never has curtains in the way. In the US most front doors are sort of in the main living room. It's an open floor plan where the entry way and living room aren't divided by walls.