Maybe it serves the same purpose as the adhesion additives that are sometimes added to mortar and concrete mixtures here in the states. I don't know the exact formulation, but it looks and smells exactly like plain old Elmer's white glue. The additive, the instructions say, helps the mortar or cement stick to existing masonry work. I suppose it seeps into microscopic voids in the surfaces of existing masonry planes and creates a mechanical bond. In the old days, bricklayers and cement workers would mix a thin concoction of cement and water and paint it into crevices and pockets of existing structures before troweling in the concrete or mortar as the project dictated. That would be my guess.
Sounds like a good guess to me, lol!