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You already know why graduated and solid ND filters themselves are so useful for landscape photography, using them in a filter holder system opens up all sorts of creative opportunities.
Only polarizer filters might be more valuable in your landscape photography camera bag. As a helpful note, many of the filter hldrs also fit circular polarizers, even allowing you to combine filters together for yet more creative control.
Learn More:ND Filters for Beginners Best Tripods Under $200 My Ideal Landscape Photography Kit