There are a few tags which don't do anything different than a div element but are 'needed' to create a semantic HTML document. My question is, how important is this?
I do agree on the importance of the use of lists for displaying stuff like product lists or menu's. A strong tag for making something look more important or using the mark tag for highlighting (found) keywords. I only don't know which advantages the div alternative elements have, like
sectionasidefooterheaderarticleWhat are the consequenses when you don't do these 'layout' semantics correctly? I've read that it has positive influence on e-readers and for blind people. But when I use a program as Dolphin Supernova to test if the sites I've created are usuable for blind people I don't see any difference between a page which has 'correct' semantic mark-up and the version where I replaced all the layout elements with the default div element. So why are these layout elements important. What are the advantages when you use them correctly. And who/what will you affect when you're not using them properly.
You need to think of it in terms of separating semantics (meaning) from presentation (design). You can create a terrible, messy HTML soup using only <span> elements, but you can make that look really pretty. The presentation will be fine, but the semantics of the source code will be terrible.
Now, to whom do source code semantics matter? To anyone who's interacting with the source code but not the presentation. This can be search engines, which try to find the most relevant content of the page so other people can find you when searching for you. This can be screen readers which try to make content accessible to people who don't care about the visual presentation of your site but need to access the content in alternative ways. Try reading an HTML soup page with nothing but a Braille display and you'll soon learn a few new curse words.
It could also just be you, trying to efficiently maintain and expand your own code over time.
There isn't always one specific benefit or penalty, but you won't be able to reap any benefits at all unless you start producing semantic HTML. The future of the web is pages on many different kinds of devices. If you ever expect to perhaps be able to see relevant information from your website on your smart watch, that smart watch needs a good way to hide all extraneous stuff like navigation and headers and zoom in on the actual content quickly. There needs to be a standardised way to identify main content and auxiliary content in order to do that, and those HTML5 tags are that way.
If you know what something means you can present it in many different appropriate ways. If you just hardcode one look, it can only be presented in that one look.
Today the only advantage may be slightly more readable source code, tomorrow the advantage will be new use cases for your content that you couldn't even think about before. You're never going to get there though if you don't take semantics seriously.
Perhaps as a more tangible example: the Opera browser had (has?) an experimental feature in which it displays an additional bar below the address bar which contains buttons to navigate the page you're on; using it you could quickly jump to the top page, the next page, an index page etc. For that to work the browser obviously needs to know what the "top" or "next" page is. That's impossible to figure out without standards. See here for further explanation.