SEO ( Search Engine Optimization ) can be improved on a site by using a variety of different tags, not just div and span.
The header tag hierarchy is a prime example of SEO and tag choice. An <h1> tag is seen as more important than an <h2> tag.
A <strong> tag will bring greater importance to set of words that you may want as a keyword. Using <blockquote> and <q> on different quotations. <big> will bring emphasis to content.