From all I've read, the main reason that some add citric acid (sodium citrate) to their soaps seems to be that it acts as a chelator which helps to cut down on the soap scum that forms in your shower/tub from using the soap. Adding tetrasodium EDTA will do that as well.
IrishLass
Edited to add- Was just doing some extra reading and found out that besides the chelating action, you can also combine sodium citrate with BHT to help prevent DOS (I don't know how much of each, though.....yet).
Also found out that there is a slight difference between sodium citrate and citric acid. Sodium citrate is citric acid that has already been reacted with lye. If you add just plain citric acid to your soap, it will increase the superfat because it will neutralize the lye some. But if you add Sodium Citrate, it will not mess with the superfat since it has already been reacted with lye. You can buy Sodium Citrate at LotionCrafters in liquid form.