Vid cards and SLI do you need it ?

Hey guys whats the reason for having two vid cards in sli is it so you can have two moniters or does it increase the performance or something
I'm planing on getting a http://www.newegg.com/Product/Product.asp?Item=N82E16814130237
and wanted to know why people are talking about it in sli so much.

The bridge between the two cards is just a small ribbon cable.  I assume the distribution of GPU load is managed by the drivers.  Other than that, just plug the monitor(s) into a VGA or DVI port on the first card.  I don't know if it actually matters which you plug it into, but if its anything like a multi-card system, you'd plug it into the "first" port on the primary card.