With structured data, you give Google more context to understand what the website is about. This can be helpful for online shops, for example, when the product descriptions are rather short and the search engine indexing is therefore imprecise.
Using structured data, additional content can also be displayed in the search results (so-called rich snippets). For example, customer reviews can be displayed in the form of asterisks or event dates. This has no immediate advantage in ranking, but increases the click rate in the SERPs (Search Engine Result Pages). The additional information makes your search results stand out from those of competitors without structured data and thus attract more attention.