Multiplexer has 3 voltage supply input pins

I'm using the 4051 multiplexer. (HCF4051B: http://www.st.com/stonline/books/pdf/docs/2053.pdfE). I'm confused as to why it has 3 supply voltage pins: (cut and pasted from pages 1 and 2 of official documentation that I linked to above):

I found this picture online from someone who seems to have connected common ground to both VEE and VSS: Post: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1238811578 Link to pic: http://xelentdesign.com/arduino4051wiring.jpg Reproduced here:

This implies that VEE and VSS can be connected to common ground, but I'm wondering why is the chip constructed with these two pins separate t begin with? In what situations would the pins be used separately?

it can pass analog signals which are often AC having a + voltage and a - voltage in reference to ground (which is usually half way tween the 2)

Why? Probably something to with handling some type of negative signal level comming into the chip. I connect the Vss and Vee together as I think on a data sheet example they are shown connected.