Professional designers think about the big picture. They create a visual language for your brand that is consistent across different contexts. Your website, logo, business cards, and even your Twitter profile have to form a coherent whole. Brands that have a consistent visual language make a more memorable impression than those that look like five different side-businesses.

The choice of fonts, spacing of text, and contrast are details that make a difference in the overall quality of your site. These kinds of nuances may not seem that important, but they affect how readable and functional the site is.