Semantic coding is the practice of separating design and structure, or separating visual appearance vs content.

Using lightweight semantic code rather than bloated nested table and div structures can actually speed up download and page rendering times, which is regaining lost importance due to mobile browsing. To put it succinctly, the code is shorter to read and understand, so that's why it downloads faster.

For the purposes of SEO, or Search Engine Optimization, semantic coding improves how easily search engine crawlers can discern the meaning of your web page. It is important to ensure that search engine spiders are seeing the content and understanding it as you wish for it to be portrayed. It should be as nice and clean as possible so that it will be easily recognized and accessed, which allows spiders to easily rank your site on search engines based on content, because they could easily understand it.

Semantic code assists screen readers which describe the page using audio for the visually impaired. Many of them rely on speech browsers to read pages back to them. These programs cannot interpret pages very well unless they are clearly explained.