HTML/CSS vs. Structured Data
HTML and CSS define HOW content is displayed. Structured data, on the other hand, describes WHAT the content means – for example, that an image is a company logo. Google can then display such a marked-up logo in different places, such as the Knowledge Graph or job listings.
The Benefit for You
Structured data helps search engines better understand the meaning of your web page content. In return, search engines "reward" this with better visibility – including in rich snippet results.
Stick to search engine guidelines! Violations can result in what's known as manual actions.