Picture from Chase p.108.
The first use of terrain models was for military purposes. The relief model allows a quick overview of a certain area. In a war, time is of essence. This means that the relief of the area of interest must be prepared before a possible conflict. However, nowadays the computer based production of terrain models allows production even in very short time, assumed the needed raw data including a digital terrain model already exist. A model of the landscape enables memorising the details much better than a map and minimises the risk of incorrect map reading. In addition the field of vision from each point is easily ascertained, something very important in war and not so easy to find out from a map. It is also possible to easily evaluate the coverage of a fire position.
Picture from Roux p.68.