A schema.org markup provides a structured way of describing the content on a webpage to search engines, which can help with search engine optimization (SEO) and improving the visibility of a website in search results.
A Knowledge Graph ID (KGID) is a unique identifier assigned to an entity in the Google Knowledge Graph, a database of information used by Google to enhance its search results. When a website includes a schema.org markup with a KGID, it helps Google understand the relationship between the entity and other related entities, which can help with more accurate search results.
Without a KGID, the schema.org markup may still provide some benefit for SEO, but it will not be as effective in helping Google understand the entity and its relationships to other entities. So, including a KGID can provide additional context and make the schema.org markup more effective.