Basics of Scene Construction - 3D Game Development Practical Introduction - page 11

Once you assign the texture to an object, Unity automatically creates a new material for that
texture and adds the material to the renderer of the target object. Materials are automatically
added to Materials folder, which Unity also automatically creates in the same location
of Textures folder. Practically, the texture cannot be directly assigned to a game object.
Alternatively, the material that is added to the renderer of the object has a shader, and the
texture is set as a property of that shader. Shaders specify the final form of the material after
applying all effects. Unity uses Diffuse shader as default. To fully understand the relation
between textures, materials, shaders, and the renderer; see Illustration 6.
Illustration 5: Adding texture files to a Unity project