The main purpose of wood preservatives is to put the oil and resins into and onto wood, which keeps out the water. Oil based products penetrate deeper and help combat weather stress better, according to the US Service Forest Products Lab and Texas Forest Products Lab. Water based solutions are all right for areas that have 13 or less inches of rain annually. You can't, because they must first be dissolved in a carrier of water and oil. It is generally less expensive, and potentially less of an environmental problem.
Some treatments only stay on the roof until the first bad rains, leaving just the pigment behind.
Also water-soluble chemicals leach off the roof over time. Tung, linseed, palm, rosewood, and soy oils are all natural oils. These oils are all a natural food source for mold, mildew, insects and fungi. The big advantage to an oil-based product is that it does a great deal to replenish the oils and resins in the wood and to protect it from moisture, sun and heat. It can be brushed on or applied with a pump-up garden sprayer, but an airless sprayer works the best.