The primary purpose of a tag in web development is to define and structure the content on a web page. Tags are the fundamental building blocks of HTML (HyperText Markup Language), which is the standard language used to create and design web pages. Each tag tells the web browser how to display or interpret the content enclosed within it.

For example, tags can specify:

1. Headings (<h1>, <h2>, etc.)
2. Paragraphs (<p>)
3. Links (<a>)
4. Images (<img>)
5. Lists (<ul>, <ol>, <li>)

In essence, tags provide semantic meaning and structure to the web content, making it both human-readable and machine-readable. Without tags, browsers would not know how to present text, images, or other elements correctly on a webpage.