Line of Sight tells you if you have terrain-level obstructions between two points. This is incredibly useful to determine if radio waves are free to travel between those two points or if they need to take indirect paths to reach the desired target point. Line of Sight does not guarantee a path for radio waves, and the lack of Line of Sight doesn't guarantee there is no path for radio waves. Radio waves crawl along the earth (ground wave), they curl around buildings (diffraction), they bounce (reflection), and they change direction (refraction). You may find yourself with perfect reception in a solid red area. You could end up with local interference or multipath in a solid green area. Use Line of Sight as just one more factor to predict reception and inform your deployment decisions.