Let's take a look at a good illustration of the explosive force of an artillery shell: the crater it forms. When the shell's detonator is configured to explode after the shell impacts the ground, instead of in the air, it can make a pretty impressive hole.
As you can see, at a range of several kilometers, the dispersion is quite high. A long barreled gun, like a ZiS-3, has a high shell velocity. It is more accurate than a short barrel howitzer (8 meters of width-wise dispersion at 3 km, compared to 12 meters for the M-30), but its flat trajectory results in more length-wise dispersion (192 meters at 3 km for the ZiS-3, compared to only 152 for the M-30). The flat trajectory is important for an anti-tank gun, however. If you are shooting at a target at a distance that is close enough, the trajectory of the shell will not surpass the height of the target, and you will hit even if you estimated the distance incorrectly. The higher the shell velocity, the flatter the trajectory, the further away the target can be while you are still capable of taking advantage of that effect. A shell travelling with a higher velocity is also carrying more kinetic energy, and is thus more capable of penetrating enemy armour.
The curved trajectory of the artillery shell was used by SPG crews in combat. Nikolai Konstantinovich Shishkin, an ISU-152 commander, recalls one such instance: "One battle was memorable. Three tanks of the advance guard left the forest and went up on a hill, where they were shot by a Tiger. There was no way to go around that clearing, and the brigade commander told me: "You are a beast-killer? Go destroy that tank." My SPG moved forward, arrived at the base of the hill, and slowly climbed up it. I climbed out of the hatch and stood up. I saw a German tank, its rear pushing against a large tree. The Tiger shot. The wind from the shot nearly dragged me out of the hatch. While I thought what to do, he sent one or two more shells my way, but missed. Only a section of the casemate was sticking out, his trajectory was flat, he could not hit me. What do I do? If I go forward, I will die. I noticed a bush on the hill. Looking through the gun barrel, I instructed the driver to move the tank so the bush lined up with the tree the enemy tank was under. I replaced the gunner. The bush is in my sights. I move the gun barrel to where the enemy's tank should be. There are a million calculations, but I spent less time making them than I spent telling you about it. Fire! I climb out, the Tiger's turret is lying next to it. Direct hit!"