Accuracy of WW2 artillery was often a CEP (circular error probably) of about 0.5% of the range, for ranges of about 4-5km and greater. That is, at a range of 10km, most rounds should hit within 50m of the target, and at 5km, within 25m. At closer ranges, the accuracy is better, but it doesn't improve as quickly any more. Of course, to achieve this accuracy, the position of both the gun and the target must be known precisely. These CEPs are comparable to the distance at which rounds are likely to kill - about 30m for 105mm, 50m for 155mm (and these rounds are dangerous to much greater distances). Where maps were good, targets can be located within 5-10m. In this case, multiple rounds aren't usually needed to make up for lack of accuracy. Multiple rounds are fired because the target is spread out - for example, a platoon might be spread over 100-200m when attacking, and an artillery battery spread over about 150m. The rounds should not be aimed at exactly the same point, but this is easy for a battery - since the guns in a battery are spread out, if all of the guns fire on the same bearing, the rounds will be spread out on the target.

The initial rounds are the most effective (if they are on target). The initial rounds can catch troops out of cover, and standing. Once the first rounds hit, they take cover, and are less vulnerable. This is why multiple rounds are fired, either at once by a battery, or in quick succession by a single gun.