The use of semantic HTML elements "enhance accessibility, searchability, internationalization, and interoperability", meaning that it makes your website to be "accessible to all visitors, to achieve a high search engine ranking, to be available to visitors from around the world, and to interface effectively with other web services."
A web that is semantically written is read well by humans and computers, allowing for easy analzying, indexing, and delivering of content, and developers have an easier time amassing information into new web services.
Section 508, an amendment to the Workforce Rehabilitation Act of 1973, was signed into law on August 7, 1998. It requires that electronic and information technology that is developed by or purchased by Federal Agencies must be accessible to people with disabilities, providing it is not an undue burden on the agency.
What exactly does it mean to be Accessible? Essentially, a technology should be able to be used as effectively by people with disabilities as it can by those without. The process may take longer than reading the screen by sight, but it must be comparable.
Search Engine Optimization is the "process of affecting the online visibility of a website or a web page in a web search engine's upnaid results, often referred to as 'natural', 'organic' or 'earned' results". Google, Bing, and Yahoo all employ the use of SEO.
There are a number of factors that play into a webpages ranking. Google, the most used search engine, uses a variety of determinants when returning search results to users, including keyword usage, user traffic, loading speed and mobile friendliness. Hundreds of parameters beyond these exist within Google's search algorithm.