Semantic elements, or giving the right hand to search engine bots

This could by trickier to explain, from a technical perspective you could always use divs for everything, I mean, it will give you a box with a full set of properties to be modified and adjusted to model your very own site, however that will get you to a very nice and taste "div soup" (Yep, it is called like that, don't take my words try google).

Despite using divs for everything will "technically" do the trick, the search engine will understand nothing about your site, then the solution to that problem is to use the proper semantic elements.

That way, not only your code will be much prettier to understand, but the engine will understand what is part of the actual article (or meaning content) of your site and what is just navigation, decoration, etc. (Google bot will love your site)