I honestly don't know. They seem inaccurate and unwieldy, they're huge, and heavy and slow down armies. They just fire 1 really big ball. Why EXACTLY were they useful?
Can you clarify your question? Do you mean “how did Napoleon use artillery effectively?”(tactics and strategy) “how do cannons help in war at the time?”(more of a mechanical question) or something else entirely?
The short answer is that what you’re describing isn’t particularly accurate of Napoleonic artillery usage. Napoleonic artillery usage piggybacked off of the reforms of Gustavus Adolphus in Sweden to lighten artillery substantially, mobilizing firepower on a greater scale than had been prior seen. This enabled him to group his artillery in grand batteries, and concentrate fire on high priority targets/restrict enemy movement. For much much more, see this detailed answer from /u/vonadler: https://www.reddit.com/r/AskHistorians/comments/373l1d/napoleon_was_greatly_known_for_his_use_of/
They just fire 1 really big ball.
I'd like to address this.
One of the best features of cannon in the Napoleonic era - and that gets used into the future even to this day - is the development of Canister Shot. The long and short of canister shot is basically a canvas bag packed with smaller metal balls, from the size of musket balls all the way up to fist sized. Fired at infantry it would act as a giant shotgun, ripping through infantry formations like it was nothing. Used against an approaching infantry column as a battery, and you could devastate the lines - this was the main thing that killed Hercules Scott's column during the Siege of Fort Erie in August of 1814.
Even the One Hundred and Third, veterans of many a hard-fought field, could not make headway against such a fire, and when about fifty yards away the column was seen by the anxious watchers in the fort to first hesitate, then waver, and then retreat in con- fusion, leaving many dead and wounded. So intense was the fire that one of the garrison compared the roar of the artillery and the musketry fire to the close double drag of a drum on a grand scale.
The guns of the fort and water battery, literally crammed to the muzzle with case-shot and bags of musket balls... opened up on them at short range with fatal effect. [...] Three captains, eleven subalterns... and 350 non-commissioned officers and men of this regiment, were numbered among the killed, wounded, and missing. The garrison along this line did not have a man hurt, and it is doubtful whether the 103rd fired a single shot in the making of this attack.
In addition, cannon existed of lighter weights: 3- and 6-pound cannon that weighed a fair amount (a reproduction 3-pdr I personally have experience with weighs about 400lbs including carriage, and I know that British brass 6-pounders tend to be in the 600lb range without carriage), but on a field carriage could easily be transported by the cannon team to be reponsitioned and reaimed as necessary. The amount of firepower a 6-pdr could bring to bear with canister shot/case shot makes it fairly effective on a battlefield, despite the weight.