Repetition of design creates a sense of familiarity for the end user or viewer. It gives the person a feeling of control over the design itself by allowing them to believe they know what to expect every time they click a link or flip the page. It also can be used to control where users focus their attention on the page.
Using consistent typography (font face, color, and size) throughout your design will help create a professional appearance. That is not to say that all your text has to be the same size. It simply means that all top level headings, all sub-headings, all paragraphs, all indented blocks, all anchor links, all bullet lists should be consistent.
Repetition of design elements helps your web pages appear more cohesive, and also improves readability as the reader comes to expect text that looks a certain way to indicate certain qualities.