Using a Graduated Neutral Density filter is fairly easy and doesn’t require any advanced techniques in post-production but the easiest option isn’t always the best choice; due to the filter’s transition being horizontal, anything above the distinction will be darkened and anything below will be left alone.
This is a good solution when there’s a flat horizon but what do we do when there are mountains projecting above it? What do we do when there are large trees in the image? Using a GND filter means that they’ll be darkened as well. That’s something we want to avoid.