The Formatt-Hitech ND hard grad 0.9 filter is by far my most used filter. As a Landscape photographer a grad filter is a must have piece of kit wether you're shooting sunsets, seascapes or landscapes.
Cameras are not able to capture the amount of detail and tonal range that the human eye can. If we look towards the sun(not directly at it obviously!) our eyes make all the necessary adjustments to balance out the scene in front of us so we are able to see detail in the bright sky and the foreground. However when we reach for our cameras to capture this fantastic scene. we end up with is a completely different to what we were seeing.
If you shoot with the camera set to auto then it will take the exposure reading from the brightest part of the scene(the sky), giving us wonderful bursts of colour in the sky but anything in the foreground will be completely in shadow with no detail.
If you are shooting in manual you can make adjustments to try and compensate to get more of a balanced picture which still has it's problems, if you expose to bring the detail out in the foreground it ends up with the sky blown out losing all detail, which makes a worse picture than having a dark foreground.
What would usually be done is to try and exposure somewhere between the light sky and dark foreground and then put it into Photoshop and spend time trying to get the balance right. A lot of photographers use this to great affect but it involves having a great deal of skill and time to use Photoshop. I prefer to get the shot right at the time of shooting and the only way to do this is by using an ND grad filter, by the time I get it onto the computer it takes no longer than 5-10 mins in Lightroom to get the look I'm after.
This shot was taken at 1/60th sec to get the detail in the foreground then using the Formatt-Hitech 0.9 Hard Grad filter reducing the light area(the sky) by 3 stops.
If you're thinking of using filters to progress in your photography i'd highly recommend starting with a ND Grad filter, very simple to use, just make your exposure highlight the foreground, pop the filter on to darken and balance out the sky. It'll save you a lot of time in post processing!!