By the 1950s, plastic had been invented and clear plastic raincoats became a fashion trend. Many illustrators were drawn to the challenge of capturing light reflecting on this new, translucent material:
Monet brilliantly captured the essence of light on water by using bold daubs of fresh paint, rather than painstakingly blending and smoothing the colors.
Briggs and Monet each realized that carefully blending with smooth brush strokes would have stripped the painting of its vitality without improving its accuracy. You have to be very, very good to get away with painting this loosely.
One other point about the illustrators who chose to paint translucent raincoats when it would have been far easier to paint a nice wool overcoat: Artists who produce art in exchange for food and shelter always develop tricks to be more efficient, save time, and (most of all) conceal any gaps or shortcomings in their skills. For example, artists who are not good at drawing hands tend to draw people holding their hands behind their backs. Artists who have trouble with perspective tend to draw pictures with a narrow depth of field. And of course, heavy shadows have long been a favorite technique for concealing a multitude of artistic weaknesses.
So I have special admiration for artists who, while working under a deadline, look for tough and interesting new artistic challenges. The centerpiece of the Al Parker illustration above is clearly the plastic raincoat. The same with the Austin Briggs illustration. These were not commercial choices, they were aesthetic decisions motivated by the same artistic ambition, pride and curiosity about the world that motivated Monet.