A human URL is the APIs.json property for each API that takes people to a landing page dedicated to an API. Often times this is redundant with documentation for an API, but ideally is a dedicated landing page to a an individual API with all of the available properties of that API available in a single click.
Think of the human URL pointing to the marketing landing page for your API. Sure, there is a documentation, SDK, uptime monitor, and OpenAPI available for an API, but provide consumers with a dedicated URL for any business or technical stakeholder to discover and onboard. The human URL is designed to drive a one-click approach to onboarding API consumers with everything they will need.
APIs.json properties are designed to store the properties of each API, but it is also meant to be a design check list when developing and evolving your APIs. The human URL is a simple reminder to make it easy for both business and technical stakeholders to onboard, keeping things simple and consistent across all the APIs are you are making available internally or externally.