What is the use of HTML5 semantic markup for intranet applications?

Question 1

There is no straightforward example to point out, but the website (even intranet) can be consumed by different user agents (on different devices).

You are probably familiar with Skype (and the iOS Safari) making phone-number-like words clickable. In the future I can easily imagine mobile browsers being smarter to assist the user in completing tasks on the page, like importing a clearly indicated contact to the address book.

Question 2

Screenreaders for blind people?

Question 3

While there is not a whole lot of immediate benefit for non-disabled people, it is still good practice. Does your company not have any externally facing sites? If it does, do those people not look at internal page code? Good practices spread just like bad ones.

see also: http://en.wikipedia.org/wiki/Semantic_HTML

Question 4

Simply said there the only rule you have to follow if you create your html documents is that it is valid html, otherwise you will have the problem that the browser would try to correct your broken syntax which may result in defects of the visual representation of your content.

In modern browsers you can use display to given any element - with some limitations e.g. with input element - the same visual look and behavior then any other element.

So if you ask what are the advantages of using semantic markup in HTML5 you should ask, why to use any of the semantic markup if it is possible to have the same result using css.

The short answer is, no one will stop you if it is your own project where you are responsible to - except the client that probably gives you requirements.

It is similar to asking: Language xyz provides comments and there is a syntax for doc-comments, but why should I use them?.

Using the semantics wisely increases the readability and thus maintainability. You are not required to use every possibility of semantics at all costs.

Using them will help you to get into the code again if you haven't looked at it for a longer time, e.g. to distinguish between the elements that encapsulate logical parts and elements that are used for styling. Especially if you use a template engine to create your code or to search for certain elements in multiple files.

Even if you are now the only one who works on the code it may happen that if the project grows that you need other people to work on the code. Or for the situation, you for some reason are not available, someone else needs to maintain your code, a good markup is essential.

Using the correct markup and additions like WAI-ARIA is not only essential for handicapped people, but also allows the browser to recognize the meaning of elements, allowing to e.g. improve the keyboard navigation. Especially in a productive environment where you need to type much, it is often faster to navigate with keyboard then using a trackpad or a mouse.