Structured data does not directly change rankings, but it influences how a page appears in results: review stars, FAQ accordions, recipe cards, video thumbnails, breadcrumb trails, event listings. These rich results lift click-through rate significantly above plain blue-link results.
JSON-LD is the format Google recommends. Microdata and RDFa are also supported, but JSON-LD is cleaner (separated from page HTML) and easier to maintain. The structured data should accurately describe what's actually on the page — misleading or fabricated markup can earn a manual action.
Google's recent guidance for AI features explicitly states that structured data is 'not required' for AI Overview eligibility. It still matters for traditional rich results and is generally worth keeping — but it is not a magic bullet for AI search.