Troops who are defending cover have a massive advantage in an assault. They can fire from their hiding place at their approaching enemies and then smite them down as they struggle to get to grips with their hidden foe. It's all in the reflexes when it comes to combat in the open; slow lumbering opponents can be quickly dispatched by a faster and more agile foe. Unfortunately, lumbering opponents, such as Orks, are tough enough to be able to withstand a vicious pummelling and come back for more.
See Cover for how the cover advantage is actually resolved on the tabletop.