Reasons you might need a storm door include:
Protecting the main door from weather damage.
Adding a layer of insulation for more home energy efficiency.
Getting more natural light when the main door is open.
Enhancing security with additional built-in locks.
Boosting curb appeal and home value.
Warning: You shouldn't use a storm door if the entryway gets more than 2 to 3 hours of direct sunlight. Trapped air heats up behind a storm door, which can damage the front door behind it. Getting a storm door with a screen adds ventilation to fix this problem.