The purpose of mixing the oxalic acid in thin sugar syrup is to make a sticky solution that will adhere to the bees bodies and not just run off as water does. The acid then will contact the mites and be absorbed/burn them, I am not sure which, or if it is both, killing the mites. The reason thin syrup is used is so the solution will be bitter and the bees will not eat it.
Go to YouTube and search for the Oxalic Acid Drench video featuring Marion Ellis for information on the why and how of using oxalic acid for varroa mites.