Reason one, DFI used the same board design for the SLI versions of this board, with little being different other then the chipset that enables SLI.
Reason two, so you can run a second pair of monitors with your setup, a total of four. DFI calls this Dual Express Graphics.
The bright side you can mod this board and run SLI. You move a jumper here and pencil in there and viola you have a SLI board. Engineers at DFI silkscreened a table right on your motherboard to aide you in your endeavors. Nvidia has done some things to make this harder, but it still can be done. Once you purchase the SLI link and go through the hastle it would be easier to buy a SLI ready board, but you can always think of it as an upgrade.