Surfacants are used in some of the herbicides. This will allow a higher degree of "mixing/application" on a whole due to a reduction of surface tention of the water.
The good thing with the Round-Up Pro Dry is the surfacant is already in the product. This gives the ability to mix to a greater/lesser concentration if needed.
Most of the herbicides require at least a 1.5 % to water mix to be effective. Often times when the 1.5 % mix is figured out between the products the price difference in almost negligable. especially if surfacant has to be added.