What are the true usages of HTML's <link> tags?

Question

Every once in a while I will see a bunch of <link> elements in code:

<link rel="Home" href="/" title="Page d'accueil de Babysun" />
<link rel="Index" href="/" />
<link rel="Top" href="/" title="Babysun / Accueil Babysun - Babysun" />
<link rel="Search" href="/content/advancedsearch" title="Recherche Babysun" />
<link rel="Shortcut icon" href="/extension/ezwebin/design/babysun/images/favicon.png" type="image/x-icon" />
<link rel="Copyright" href="/ezinfo/copyright" />
<link rel="Author" href="/ezinfo/about" />

The link element is described as:

The <link> tag defines the relationship between a document and an external resource. The <link> tag is most used to link to style sheets.

(emphasis mine)

What external resources? SEO?

If so, what does Google care about:

<link rel="Home" ...>
<link rel="Index" ...>
<link rel="Top" ...>

Answer 1

as the spec defines it:

...it conveys relationship information that may be rendered by user agents in a variety of ways (e.g., a tool-bar with a drop-down menu of links)

further on, you can read about link types, and extract some specific information on this.

Answer 2

You can read a bit more about the tag on this pages:

http://www.w3.org/QA/Tips/use-links

Google, for example, uses the rel="canonical" to identify each one of the pages with the same content should be indexed by search engines.

http://support.google.com/webmasters/bin/answer.py?hl=en&answer=139394