"System Tray" icons were a great idea in Windows 95, but over the years the concept has been abused. It's not uncommon to get a new Windows PC with a dozen system tray icons, usually serving no purpose other than to advertise the existance of the application to which the icon belongs.

Linux machines are not quite as bad, but this hasn't stopped the designers of Gnome 3 from hiding these icons in a hidden bottom bar that appears only when you move the mouse to the bottom of the screen.

If you still prefer to have these icons on the screen (and lets face it, the Gnome 3 top bar is mostly empty space anyway), you can use the icon manager extension to selectively move icons from the bottom bar to the top bar.