I keep seeing role attributes in some people’s work. I use it too, but I’m not sure about its effect.

For example:

<header id="header" role="banner">
    Header stuff in here
</header>

Or:

<section id="facebook" role="contentinfo">
    Facebook stuff in here
</section>

Or:

<section id="main" role="main">
     Main content stuff in here
</section>

Is this role attribute necessary, better for semantics, and does it improve SEO?

A list of roles can be found here, but I see some people make up their own. Is that allowed or a correct use of the role attribute?