What is the difference between a neutral density filter (aka ND) and a graduated neutral density filter (aka GND or Grad ND)? An ND filter will provide a uniform reduction of light throughout the entire filter. A grad ND will start with nothing on one end and slowly graduate to darkness on the other. One of the most common problems in landscape photography is having a sky that is too bright while a landscape is too dark. This is where the Grad ND is most useful. As only half of the filter is dark, you can position the filter so that it only covers the sky portion of the image while the clear area is on the dark landscape. The result is a perfectly (or nearly) exposed image where the clouds have texture as does the landscape. This is a fairly easy thing to do if your landscape photo has a straight horizon like in this image.
However, if there are mountains in the image, then you will notice that the tops of the mountains will be a bit dark like in this image.
One of the most obvious uses of this filtering can be seen in the movie Top Gun - yes the Tom Cruise movie of the 90s. Although they did not use the a Grad ND filter per se, they did use a Grad warming filter. In other words, the dark area was not just gray but orange in color. The purpose of a grad warming filter is to simulate a sunset or sunrise. The problem with these filters is the same as a grad ND filter. Unless you have a very straight horizon, it becomes obvious that a filter is being used. In the Youtube video below you will you will notice the use of the filter at minute 1:59 and 2:49 in particular . Notice that the upper part of the frame is a deep orange color and that color creeps into the people and the airplanes on the deck. This does not happen naturally. The upshot is that the vast majority of viewers would never notice this.
In conclusion a graduated ND filter can be useful, if used properly.