Remember who private collectors are. They're the shrewdest and most successful business people, ladies and gentlemen of good real estate and rolling stock who seldom part with money unless for some clear benefit. When they buy works, their capital vouches for art in general. By their purchases, new art, especially, is understood to be worthy of investment. The only reason more wellheeled people don't follow their lead is that they don't feel sufficiently informed or secure in their tastes or historical perspectives.
Some collectors are magnanimous and patronise art for the sake of cultural health.
Some have no philanthropic motives and jealously monitor the appreciation of their collection. On a symbolic level, however, their contribution is identical. They tangibly assure the community that there's some value in new art. Symbolic is the right word. The sums shouldn't be overestimated.