The right element gives you accessibility free. A <button> is focusable, activates on Enter and Space, and announces itself as a button. A <div onclick> does none of that, and recreating it takes four extra attributes people rarely add.
header, nav, main, aside, footer. Screen reader users jump between these the way sighted users skim. A page built entirely from divs has no landmarks to jump to, so it must be read from the top, every single time.
One h1 per page, then h2, then h3, never skipping a level because the size looked better. Size belongs in CSS. Screen readers offer a heading list as the main way to navigate a long page; a broken outline breaks that.
Semantic markup also gives search engines structure, makes your CSS selectors saner, and gives you free keyboard behaviour. Doing it properly is less work, not more.