system
A chat on subreddit /r/NoStupidQuestions
human
Those are called storm doors. Having a storm door allows you to leave the front door open to let in more light, and also can help protect your primary door from rain, ice and snow, reducing the costs of maintenance and helping it last longer.

A lot of storm doors also allow you to easily swap the glass for a screen to allow for extra ventilation when the weather is nice.
gpt
Also the storm door adds extra insulation around the main door during extreme temps.