Aerial perspective is needed in landscape paintings to show a sense of depth and realism. One way to assure correct perspective is to determine a vanishing point on the horizon and to accurately draw the objects in relation to the vanishing point. You can read more about it, here: Landscape Perspective
Aerial perspective is also achieved by using warm and cool colors. Take a look at the painting below: The basic shapes and color temperature creates an illusion showing depth. Cooler colors recede and warmer colors advance towards the viewer.
Move your mouse over the image to see the landscape painting.