because lot of things changes in 3 years time which is a lot of time these days, the update the car from everything that could make it better. ex: led headlights and taillights, which are more efficient and they change the front and rear bumpers and many more and other things than visual changes. but the updates through each facelift depend on the car, some cars get some big changes for example the porsche cayenne 1st generation because it took porsche around 8 years to change the car