You could build an entire website using only <div> and <span>. Browsers would render it fine. But screen readers would see an unintelligible wall of boxes, search engines would struggle to identify what's important, and your team would have to read every CSS class name to understand the document structure. Semantic HTML fixes all three — for free.
1-A 2-B 3-A 4-A | 5 footer 6 semantic/clear 7 = <header> and <nav> 8 = Better rankings/snippets; screen-reader landmark jumps 9 = Styling/layout wrappers with no meaning | 10 = header + nav + main (article/section + aside) + footer