An army wins by concentrating its main force at the decisive point and moment, not by spreading it thin.
Jomini studied Napoleon's campaigns and drew clear rules about where troops should be placed on a map. He insisted a general should always mass their main strength where it could strike decisively, instead of spreading it thin along the whole front. His rules were more geometric than Clausewitz's.
Jomini was not only talking about soldiers on the battlefield. He applied the same logic to war strategy as an engineer designing a bridge: every piece must be exactly where it bears the weight. If you spread resources thin, the bridge collapses. For example, an entrepreneur launching a new product should not advertise in ten small towns, but concentrate the budget in one big city. There they can test and adjust quickly. This idea influenced not only wars, but also modern management.
Why it matters His rules about concentrating force were taught to generations of officers in military academies.