These are two key elemets , as important as composition . Knowing what to focus on and the how to make it look good. First , we need to understand how DOF ( depth of field ) works. There are two measurements in a landscape you need to consider , from your camera to the point of infinity.
What landscapers attempt is to get this whole frame to look sharp and clear. A small aperture will achieve this , but you dont actually need to always go to the lenses maximum aperture as this may start to show flaws in optical quality.
What you are looking for is a balance . Now as a general rule of thumb , DOF works one third back from the point you focus on and two-thirds behind it. So focusing on the horizon actually waste loads of DOF.
Now , you can see why your cousing point is important as choosing the aperture value.