For the same reason we should be thinking about buying the new P90, FN2000, SCAR, AUG (maybe), etc.....if we show the gun manufacturing community that they CAN and WILL make money by introducing "evil assault weapons" into the US civilian market, it will only open the door for more of the same in the (then very bright) future. The only thing about the Sig is that it is probably overall much more anticipated than the other possible "assault rifle" releases, meaning that it's all the more important that the 556 be a total success.
If, however, the 556 (and any of the other "big dogs" for 2006) flops or is only barely successful, we will have more or less sealed our fate for the future of "assault weapons" releases by major gun companies. Not totally, of course, but it will definitely make a bad impression on any manufacturers trying to decide if introducing a new evil black rifle is worth the risk or not.
It is actually pretty important that the Sig 556 (and "the others") succeed to some extent. Fortunately, I think the 556 will do at least pretty well.