Concrete is a natural insulator and will stabilise the temperature of its contents. This stability makes for a smooth and gradual fermentation, because there are no temperature fluctuations. For the same reason, concrete is also ideal for storing and aging wines.
The concrete tanks are also porous and that's where it beats stainless steel. With a gradual introduction of oxygen, the wine can breathe and evolve. Wine fermented in concrete has the round mouthfeel of wine fermented in oak, but it has much greater purity of fruit flavor and intensity of color.