Airing down to a psi in the low 20s from high 30s or 40s makes plenty of difference and gives plenty of benefit. In addition to increased traction, it just plain makes the ride more comfortable. If you're stuck in deep sand or gravel or trying to crawl over rock obstacles, then airing down all the way to single digits might be needed but I wouldn't advise driving for miles and miles over multiple days like that. Our off roading in Arizona can have you out for days at a time covering a hundred miles with lots of sections near highway speeds. I haven't aired down below about 18-20 psi in years. If my tires are spinning I just break out the winch or the maxtrax.