What is special about the hicolor icon theme?

Googgling around I found this:

In order to have a place for third party applications to install their icons there should always exist a theme called "hicolor"

• This name is chosen for backwards compatibility with the old KDE default theme.

The data for the hicolor theme is available for download at: http://www.freedesktop.org/software/icon-theme/. Implementations are required to look in the "hicolor" theme if an icon was not found in the current theme.

More at Icon Theme Specification (Gnome)/Directory Layout.

And from the package description:

So, yeah it's the fallback icon theme.