Filters have always been, and will always be, an essential part of landscape photography. Photoshop enthusiasts may beg to differ, but the soaring demand for these products over the past few years proves otherwise. You can, to a great extent, get away with not using filters, but simply ask yourself this; do you prefer spending time in nature or behind the computer? If you answered nature, then you need to look at getting filters.
The primary reason for using a filter system is that the sky is usually brighter than the land, especially in dramatic sunset light. Graduated ND filters are dark on the top half and transparent on the bottom half. When the dark part is positioned over the sky of an image, it ‘reduces’ the amount of light allowed through and this results in a darkened exposure of the sky. This concept is displayed as simply as possible in the image below. On the left it shows the effect with no filter while on the right it shows the effect using the filter. Pretty awesome.