Can you be a little more specific? Much of the modern military structure can be traced back to the Napoleonic era, but I'm not sure what you mean when you ask why.

A little clarification on your question would be nice.

But generally, there are two reasons why military units are the way they are. The first is a matter of control and the second is a matter of tradition. Firstly, divisions (not the unit) exist within militaries as a simple matter of control. Men are grouped under an agreed upon leader who tells them where to go and what to do. The number of men placed under a leader is determined by the number of soldiers they can effectively control during battle. And then the number of leaders who controls sub-leaders, etc. are based off the same thing. So the ideas of military units really come from how effective a commander can command units.

The second reason is tradition. Armies, generally, are extremely tradition bound, and most militaries look back to their successful predecessors to come up with ideas for the future. In this sense, armies have created units to emulate the successful predecessor of all European armies, the Roman Manipular Legion. Or, I should say, the Roman Manipular Legion as interpreted by a bunch of 17th and 18th century military theorists. Basically, what the Roman Army did really effectively was that it created a group of units of a roughly standard size, and roughly standard equipment, which was effectively controlled by a standard number of officers. This meant that an officer could request the help of a legion, or a cohort, or a century, and know exactly what they were getting. This was interpreted by Dutch, Swedish, and French thinkers throughout the 17th and 18th centuries. These thinkers readopted the concept of the Roman Legion and renamed it the "Brigade." in France, these Brigades were combined into a division, which was itself combined into a corps.