Semantic Markup makes websites more meaningful by helping with communication. It uses elements such as p which is for a paragraph, or h1-h6 for headers.
Section 508 of the Rehabilitation Act states that information on certain wepages must be accessible to accomondate for certain individual's disabilities. You must use visuals that eveveryone can see, and add alt text to your visuals. You shoul make your website keyboard friendly,and be able to have files open for everyone and for them to be able to use them in different ways, such as when using screen readers.
Search Engine Optimization is where you get more site visits, by using things that users are looking for. You should think of your your website looks to the average user, and make your website unique and rememberable.
Semantic markup makes things easier for people with disabilities to read your webpages. It also helps search engines find the important information on your page. By including that, you will increase users to your page.