When it comes to 3d modeling, textures are very important if you wish to add detail without a lot of extra work. To actually sculpt out and model fine detail would not only be a waste of time but also a waste of computer resources, which it much more of an issue in games which are dynamically rendered. With a texture you can project an image onto a flat surface, creating the illusion of a much more complex and believable object. Instead of creating a realistic brink, then repeatedly copy and stack it to create a wall or building you can simply create a simple wall or building and slap a brick wall image onto it. Now up close you could see the difference between the two but further away no one would be the wiser. The images above are just three random ones I found online that show a little of the neat things textures can be used for. The first is a texture by itself, a very complex one with a lot going on and a great feel of depth and details, a level of detail that would take an extremely long time to match by modeling. The second image is just a tiled checked board pattern, not nearly as complex as the first but still adding a lot to the object its attached to. The third is an example for something you would find while playing a video game. It appears much more realistic but it does so without creating too much extra work for a processor to calculate.