In Napoleonic combat, there were two main formations for infantry, the Line and the Column. In Line, a unit was only a few ranks deep but had a long front facing the enemy, while in the Column, the unit had a smaller front but many more ranks. The Line gave greater firepower but, as there were fewer ranks behind the front, it was easier to rout. Further, the Line was not as useful for charges and could be easily broken through, but was less vulnerable to artillery. The Column, on the other hand, was the ultimate charging formation as the weight of the column would push the front through, but the unit sacrificed firepower and was gutted by artillery.
In Warhammer strategy, the same rules apply. A unit's firepower is a factor of how many models can attack, but as each rank adds +1 to combat results, the deeper the unit the more damage it can take. A commander must weigh the added firepower against the disadvantage of possibly losing combats and, as a routed unit is often a dead unit, most commanders opt for a deep formation.
While a four rank column is great for moral, it is a tempting target for artillery. Every artillery unit is capable of dealing great amounts of damage to a dense packed unit. From cannons and bolt throwers to rocklobbers and death rockets, each of these weapons has an area of effect which is either a line or a circle. Every model in the area of effect is vulnerable to the attack. If a unit is in a deep column, then it is more likely that a hit will kill a few models; however, if the unit is in a two deep line formation, even if it is twelve models long, fewer models can be killed.
So if artillery is causing you a nightmare in the Warhammer world, take a note from Wellington and spread your units out in Lines.