5D Mk III, EF24-70 f/2.8, Lee Big Stopper. F/11 at 21 sec.
Neutral density and polarizing filters are about the only filters really needed any more for landscape photography, there are a few exceptions, but currently these filters dont really have an option that can be done in camera or in post.
ND filters allow you to control the amount of light hitting your sensor, along with aperture control, you can allow the right amount of light to do exactly what you want in most lighting conditions.
This allows us to shoot in bright light and get the feeling of motion in our images, there are many uses for this; smoothing of water, motion of a running animal or person, motion in vehicle traffic, abstract images and more. I arrive at a shoot long before the golden hour, its nice to be able to start my process before the light dims enough for normal shooting and allows for far more abstract images than with out an ND filter.
Frequently I cant shoot slow enough when at an ocean front shoot, with ND filters I can shoot at 30 seconds on a bright day, allowing me to get in more shooting time as the evening progresses, I can maintain long shutter speeds or change to a different filter to speed up my shutter. Varying the aperture and the filter density allows me to select the exact speed for the effect I'm looking for.
These are the ND filters I use regularly.
Lee Big Stopper
Lee Litttle Stopper
Lee 0.9 GND Filter
Lee 0.6 GND Filter
Lee foundation kit
Shutter speeds with Big Stopper
Images In Light