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.

ok thanks, but do you just plug your monitor into one of them or is there an adapter or somthing? I've never used or seen up close
an SLI configuration.