Extra devices like a Blu-Ray player, PS3, Xbox360 or HD Digital/Freeview box.
And since many modern PC graphics cards these days support DVI-HDMI or native HDMI connections, you can even have a second or third PC up especially if one takes up each connection, VGA, DVI & HDMI.
If the monitor has speakers (Which are usually crap anyways!), then you will get sound through the HDMI cable too.
And if your monitor supports it, HDCP content protection.