The role of artillery is very simple - artillery kills. Anything that sits still long enough to be targeted gets destroyed. One of the main purposes of encircling a foe is to pin them down for the artillery. As far as results go, the differences between this and an airstrike are minimal - but important: Artillery fire is easily corrected, wheras an aircraft must make an entirely separate pass. Artillery, being ground-based, has unlimited loiter time. On the defensive, artillery rounds can be pre-registered, which means that they can hit a specific spot on command. Artillery rounds, being more plentiful than bombs (if smaller); can be used for speculative fire and against targets of opportunity, oftentimes targets as small as individual soldiers (there could be more nearby). Armor, by contrast, has an entirely different mission. Artillery normally sits somewhere behind friendly lines and waits for targets to be called in, and them blasts them from up to 17 miles away. Armor is supposed to use their mobility to go around the enemy or through their weakest point and then run rampant in the enemy rear, where anti-armor weapons tend to be scarce. Armor can also be used to destroy enemy strongpoints directly, especially in built-up areas. Note that tanks, being equipped with radios, can and will call for artillery support.

Artillary is all about support of the ground forces. Armor and Infantry are still the only two players on the battlefield who can go and seize a piece of terrain. Tube artillary is designed to make their job much easier by killing the enemy before he can kill your ground forces or by keeping his head down so our ground forces can get close enough to kill him themselves. All three of these individual units are capable and dangerous but would be utterly torn to shreds without the support of the other two if they tried to win the battle alone.

THERE IS NO CHEEPER WAY TO DESTROY MORE ENEMY IN CONVENTIONAL COMBAT THAT THE QUEEN OF BATTLE!

Bioweapons. :P But we don't use em, so artillery I guess.

There are two generally accectped classes of weapons on the battlefield: direct and indirect fire weapons. Tanks are in the former, artillery and mortars are in the latter. Direct fire weapons are those where the gunner can see the target, the gunners of indirect fire weapons do not and use an artificial aiming reference. The reason both are needed is that when a unit gets engaged with the enemy, it's freedom of maneuver is limited because of enemy fire. At that point, indirect fire weapons, not in direct view of the enemy, are brought to bear on enemey positions providing lethal firesupport. This notion has been around since WW1.

I have before used the simplified parallel: The infantry, cavalry and artillery is like the old game of knife, stone, bag: The knife cut the bag, that bags the stone, that dulls the knife. cavalry kills artillery, that kills infantry, that stops cavalry. It is very simplified, but in my view serves as a first approximation and an underlining of their mutual interdependece.