i think it is so that activity can be measured around the country - especially in swing states; and so that any comments left by the visitors can be summarized by area of the country in order to measure the degree of how vocal folks might be. it is info that can supplement polls that are being taken and indicate where some should be taken

summarizing by zip code also enables the obama organization to draft specific e-mails back to those who visited/posted rather than send out just the generic campaign messages.

just my opinion