Have you ever looked through a newspaper or magazine? More and more people go online to read the news these days, but printed news can still be spotted at the store, in newspaper vending boxes or on your neighbor's porch.
If you look at the front of a newspaper, you can see text throughout the page with different visual treatments. There's usually a big headline at the top. Columns are filled with paragraphs telling news stories. A solid line travels across the page to divide it into sections. These things are done to organize content and create a visual hierarchy, giving readers a clear path to get to the information they want.
It stands to reason that text content on a website would require similar treatments to create a logical and visually pleasing layout. That's why HTML includes a number of tags to categorize, organize and format text content.