HTML text tags indicate the structure of HTML pages; that is, they indicate what role a text segment is playing in the page organization. Using these text tags you can indicate which text segments are headings, subheadings, lists, quotes, and paragraphs.
Phrase elements "add structural information to text fragments," according to the W3C. These elements may seem to be adding visual style to your pages, but in fact they're adding structural information. The text that's placed within phrase element tags will be treated differently by screen readers from text that has purely visual styles applied; the tags indicate something about the nature of the text itself beyond its appearance.
Dan Cederholm's excellent book, Web Standards Solutions: The Markup and Style Handbook, has an entire chapter on phrase elements, with styling suggestions that can be added to your style sheets.