In the context of Page Designer in Salesforce Commerce Cloud, the JSON metadata definition file serves critical roles in defining the structure and capabilities of a new component. Firstly, it defines the regions within the component type, which are areas where other components can be nested or placed, essentially outlining the structural layout of the component. Secondly, it specifies the attributes that a merchant can set when configuring the component in the Page Designer tool. These attributes can include various configurable options like colors, sizes, or text, which allow for customization of the component's appearance and behavior without hard coding these properties. This file does not directly define the rendering logic or the responsive layout, which are typically handled by the template and CSS respectively.