As Jac Weller wrote in Wellington At Waterloo, the Napoleonic concept of a corps is that of a force large enough and with diverse-enough composition to function as a mini-army. In other words, a corps features its own infantry, artillery, and cavalry under a single commander, functioning like a mini-army. Corps can either act on their own or be combined for larger engagements. When on the move, corps were to move on parallel routes close enough to come to the aid of another if attacked.

Some advantages provided by this organization were as follows:

• Less strain on the army commander: rather than having all your divisions to worry about, an army commander can focus on their corps commanders. Since multiple divisions go into each corps, this reduces the management load considerably.
• Better logistics: by spreading out as mini-armies rather than staying as one large one, corps can spread their logistical footprint over a wider area, whether that means a larger area of land to loot/forage from, marching down multiple parallel roads rather than a single one, etc. By being large enough to hold their own in combat and close enough to support each other, they are not as vulnerable to being defeated in detail.
• Allows for independent missions and more complex strategies: since corps can act independently, you can have them fulfill one task while the rest of your army does another, such as the Prussian III Corps fighting the Battle of Wavre by itself, thus keeping Marshal Grouchy's forces from joining Napoleon at Waterloo or continuing to pursue Blucher.

Since the Napoleonic era, the concept of a "corps is a mini-army" has become outdated as more diverse groups of weapons filter down to smaller and smaller units. A Napoleonic-era British infantry regiment of 500 people would consist almost entirely of soldiers with muskets. Infantry units might operate alongside or just behind friendly artillery as John Keegan notes in The Face of Battle, but these are not part of the regiment in the way that a modern mortars would be "organic" to the heavy weapons platoon of a modern infantry company.