Why do you HAVE to load if you get into competition ?
Here is my take on that:
1. Due to the sheer volume of rounds fired for training and practice, reloading helps save money.
2. When reloading, you can tailor your loads. For instance, a lighter bullet will travel faster and have a flatter trajectory to punch paper nicely, but may not have the ompf! to knock over a steel plate. So make a batch with a heavier bullet, or push the bullet to a higher velocity by increasing (within safe limits) the chamber pressure.
3. A hollow-point round will generally perform better (in terms of expansion and weight retention) when pushed at higher velocities, so you can tailor your defensive loads as well.
4. Some off-the-shelf ammo burn with enough residue to make cleaning a miserable task, and shooting it can create a cloud of smoke. The freedom to choose what gunpowder you use can eliminate these concerns.
5. A firearm with a shorter barrel (like my G26 and G30) will fail to develop maximum muzzle velocity for factory ammo simply because the bullet will exit the muzzle before all the powder is combusted (also creating a dazzling flash at night!). So I load using fast-burning powders and guess what? Minimal flash, and target velocities achieved! However, in a firearm with a longer barrel (like say a Beretta 92), this may result in unsafe or dangerously high chamber pressures, so again, the reloaded round is tailored to the firearm.
And then there is the matter of energy transference. When a bullet at warp speed punches a neat hole in paper, the bullet transfers little to no energy to the paper, and keeps going until it hits the berm and buries itself in the ground. For full transference of energy, as in a bullet hitting a steel plate, the bullet hits the target, transfers its energy to the target, and stops there, knocking the target down.
Most 115grain 9x19mm bullets may have problems knocking down steel plates, so when I shoot IDPA I actually use 147grain bullets at higher velocities.
Impact energy is roughly gauged as equivalent to the bullet weight (in grains) multiplied by the bullet's speed (i.e. actually, the bullet's velocity at a point usually 10 feet from the muzzle, in feet per second). Strangely enough, Einstein's expression for Energy is almost the same: Energy = mass multiplied by speed (E=mc2 means Energy = mass times the speed of light, squared).
In competition, there is something called the Power Factor, that is used as an aid in classifying certain firearm calibers. Power Factor (or PF) is computed as the bullet weight multiplied by the bullet speed, divided by 1,000.
Both IDPA and IPSC/USPSA require that participants use firearms with a minium PF of 125,000 (for IDPA) and 125 for IPSC (the same number, one is divided by 1,000 and the other is not). In IPSC, there are two power classes, Minor PF (anything over 125 and under 165) and Major PF (anything over 165) so if one had a fancy 9mm and wanted to compete in Major, he would need to load heavier bullets flying out of the barrel faster.
In practical terms, look at it this way: an 88grain 9mm bullet travelling at 1,300fps has a PF of 114, can't compete. A 115grain bullet at 1,300fps has a PF of 150, and the same bullet travelling at 1,500fps has a PF of 172. A 147grain bullet zooming at 1,800fps has a PF of 265.
So in practical and mathematical terms, the ability of the reloader to customize his loads (for plinking and for competition), as well as to tailor his loads to a specific firearm and target combination, as well as preferred shooting style, are flexibilities afforded to him by reloading and not open to him when using off-the-shelf ammo.
And how does the reloader determine the combination that works for him? Lots and lots of reloading and testing sessions. And when he finds a combination that works well for competition, he will wisely load up and stockpile that ammo, both for practice and for actual competition.
At least, that's my logic on it.
Cheers!