Design, and especially web design is not just about making things look pretty. The aesthetic does play an important role. However, design is mostly about function. The basis of design is making the interface function so well that the visitor would never stumble. A good user interface generally starts with notions of visual hierarchy. Visual hierarchy basically makes use of colors, contrasts, shapes, position, orientation, size and texture to organize elements of a page in such a way that it determines visual important.
You might want to take the test on yourself by looking at any given web page and seeing where the read naturally go. Sometimes when elements are too uniform on a page, there is no motion shown and therefore the eyes are not guided anywhere. Same goes for when all text are highlighted or all of them are in bold, the special character is not really eye-catching.
Generally, factors to consider in order to achieve an effective visual hierarchy are diverse. For instance page scanning patterns would dictate how the page is read and in what sequence. The size and typeface weight and pairing is also determinant in grabbing the attention of the reader.
Visual hierarchy is yet another tool for the web designer. However, it might be one of the most important ones. It helps to analyse the function and flow of the web page rather than focusing on the fancy graphics that might not necessarily be what the user is looking for. The purpose of each page element comes first.