Putting a wine through the press separates the liquid from most of the solid matter. The choice of when to press depends on how much color, flavor and tannin the winemaker wants to extract from the skins. The juice that will drain off by itself without pressing, called free-run, is the prime stuff.
Once the press begins squeezing, it extracts more tannin and other materials that can roughen the texture. Because the pressed juice contains much more tannin and is a completely different style than the free-run juice, it is aged separately and is either used for blending later or for producing a wine of lesser quality. This is true in white wine and red wine, the whites preceding fermentation and the reds after fermentation.