Foreground Interest
An element placed close to the camera to anchor the composition and establish depth.
In depth
A photograph is a flat surface, so depth has to be constructed. Including something near the lens gives the eye a starting point and a scale reference, and the size difference between it and the distant subject creates the impression of space. Wide-angle lenses exaggerate this because they allow the foreground to be very close indeed.
Examples in practice
Rocks half a metre from a 20 mm lens give a mountain range behind them a sense of distance the empty view lacked.