So you've got your battlefield, but it's flat, featureless and devoid of life. While this might be appropriate if you're fighting over a starship landing field or deserted salt flats, it makes for a rather dull battle. Terrain is an important feature on all battlefields. A good commander will use the terrain to his best advantage, blocking the enemy's lines of fire, getting his heavy weapons into good firing positions and concealing his troops until they are ready to strike. Troops on the ground need cover and concealment if they are to survive against the deadly weaponry of the 41st millennium. A very large terrain piece can become the focal point of a battle, such as a bridge that must be taken or a refinery that must be destroyed.
Terrain
Source: Core Rulebook 3E, p. 123
Terrain lets a commander block enemy fire, find good firing positions, and conceal troops, turning a flat table into an interesting battle.