well...
putting two cards in sli gives a performance increase. sometimes the performance of two lower end cards in sli is better than one expensive card.
for instance putting two 660ti ($250+$250) can offer performance numbers similar to a titan ($1000) or 780ti ($720) for less money. of course if a game doesnt support sli performance will be much lower and in some cases running with sli could be slower than a stronger card.
sli also offers more ports for connecting up multiple monitors