There are a number of blending drivers, as indicated below. While varietal blends tend to dominate, blending can occur down to the single-variety, single-plot level where free-run and press juice are kept separate and then blended in a winemaker-determined proportion at a later date.
Benjamin Lewin MW illustrates this difference in his discussion of mono-varietal Cabernet Sauvignon versus Cab-dominated Bordeaux blends (Cabernet Sauvignon and its blending partners, Tong #15). According to Lewin, the mono-varietal Cabernet Sauvignon shows greater purity of fruit in its youth but is linear compared to the blend which exhibits a broader flavor spectrum. This is illustrated graphically below where varieties are represented on the x-axis and blending factors (primarily terroir characteristics) are shown on the y-axis. A mono-varietal with two blending characteristics (free-run and pressed juice, let's say) will have less complexity potential than a two-variety blend which will exhibit dual-variety characteristics plus terroir characteristics associated with each. For a Bordeaux blend the potential is magnified.
But, according to Lewin, it is with age that the differences between the wines really appear. The mono-varietal Cabernet Sauvignon matures but the purity of fruit of its youth yields to austerity while the blend produces the "savory notes of tertiary development."
Eben Sadie views the New World's obsession with varietals as detrimental to progress on the complexity front as producers flit from varietal wine to varietal wine based on "fashion." Lewin sees a New World attitude typified by "blending is what you do only when the pure varietal wine wouldn't be good enough." They both view complexity as high on the desirability list and tightly (but not exclusively) linked to variety blends.
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