Yes we use texture for making realistic pictures because almost all skin has a certain level of bump to it. If you were to look closely at your own skin, you'd see it's not perfectly flat. Painting in pores and different bumps can really help push the realism of the model. Your first thought may be to establish the diffuse color, but first creating the small details is usually best to do first because these things can change the look of the diffuse. It's also important that you don't overdo the bump for your texture. The bumps and pores on skin are typically very subtle so make sure you're creating the bump with some caution. It can be very easy to overdo a bump map and creating a skin texture that is too bumpy can create some very unrealistic results.In this picture texture is used in very good way and very properly.
