How structured data works
A page is written for people, so a search engine has to infer what it is looking at from headings, wording and layout. Structured data removes the guessing by stating it directly: this block of text is a recipe, this is the author, this is the opening time, this is the service area. The statements sit in the page’s code, invisible to the visitor.
Two things have to line up. The vocabulary — the agreed list of types and properties, published as schema.org — supplies the names for what you are describing. The format supplies the way those names are written into the page. JSON-LD is the one Google recommends, and it sits in its own script block, separate from the visible markup, which makes it far easier to maintain than the older approaches that wrapped attributes around the content itself.
Why structured data matters
Adding markup does not lift a page in the ordinary results. What it does is make a page eligible for a richer presentation — the extra detail that can appear beneath a listing — and eligibility is not a promise, since search engines decide case by case whether to show it.
The less visible benefit is becoming legible to machines generally. Assistants, aggregators and the systems that assemble answers all prefer information they can read without interpretation. If your opening hours, location, services and identity are stated plainly in markup, you are easier to represent correctly and less likely to be described from something stale or second-hand.
Where structured data goes wrong
The rule that catches most sites is that markup must describe what is actually on the page. Marking up questions and answers that a visitor cannot find, prices that appear nowhere, or ratings that exist only in the code is a guidelines breach, and it can cost a site its eligibility for rich results altogether. Search engines also decline some categories outright — reviews of your own business published on your own site being the usual example.
The quieter problem is accumulation. A theme adds markup, then a plugin adds its own, then someone pastes a block into the head, and the page ends up describing itself twice with conflicting details. Markup left over from a page’s previous life is just as common: an old event, a product that was discontinued, an author who has left.
How to act on it
Start with what you genuinely are, not with the list of types that produce the most decorative results. An organisation, its people, its services, its location and its articles cover most sites, and describing those accurately is worth more than reaching for a type that does not fit.
Generate the markup from the same source as the visible page wherever your platform allows it, so the two cannot drift apart. Validate before publishing with the Rich Results Test and the Schema Markup Validator, then watch the enhancement reports in Google Search Console, which flag errors after the fact. If you are pursuing a specific rich result, read the requirements for that type first — implementing schema markup without checking the required properties is the most common reason nothing appears.