An individual contributor can be a master of a single stage of a project—brilliant at design, fast at coding, or meticulous at testing. A leader, however, must navigate the entire journey. They must be able to take a project from a vague, ambiguous idea all the way to a successful launch and beyond.
This end-to-end journey is a test of a huge range of skills: technical design, project management, communication, stakeholder management, and team motivation.
Your answer to this question is your opportunity to provide a "case study" of you as a leader. It's a chance to show them not just what you can build, but how you lead.