From Chevron;
"Aftermarket Additives ; It would be convenient for the user if a finished diesel fuel could satisfy all his or her requirements without the use of supplemental additives. Although this is most often the case, some users must use additives because the low temperature conditions in their region are more severe than those for which the fuel was designed, or because of other special circumstances. Other users feel that they need a higher quality diesel than regular diesel. And, finally, there are users who regard the cost of an additive as cheap insurance for their big investment in equipment.
A large number of aftermarket additive products are available to meet these real or perceived needs. Some are aggressively marketed with testimonials and bold performance claims that seem "too good to be true. " So, as with any purchase, it is wise to remember the advice, caveat emptor – let the buyer beware"