Accessibility. Screen reader users navigate the screen and jump between headings using various keystrokes. If you do not use standard h1, h2..... etc tags then this form of navigation is removed.
Ask Amazon about it, they got sued for not keeping up with accessibility a few years ago. Nice lump sum settlement too.
Let me guess.... They were sued in good old USA, right? I couldn't care less tbh and I'm using my own tags for years allready in websites I design for my customers. And tell all the ones using flash for menus etc about it aswell.
