From runners and drummers to semaphores to megaphones and radio men, and army has to have its lines of communications at the front line and behind one or both sides of the frontline.
As far as unit structure goes, it thank that a set of 40 to 50 figures should be able to create a typical platoon consisting of:
1) HQ: Junior Officer, Senior NCO or platoon sergeant / warrant, Platoon Radioman/Standard-bearer and some support weapons team like MG, mortar, cannon, catapult or archers. One of a kind poses
2) Section/squad command figures of tow to three poses at one to three or four of a kind. These, would include section level radiomen, buglers, drummers, medics, small flags or standards etc.
3) There would be three to four sections, one section involved in advancing, one in firing and one defending. Possibly, have one extra section on guard or at rest. I’d say one to three poses of four to eight of a kind for each squad.
4) Maybe a few extras like casualties, senior officers, generals or personalities to complete the set also make it a flexible enough unit, for building up larger units like companies or battalions buy buying identical or complementary sets.
I'm simple collector and occasional modeler, I judge most sets by how well they approach my four standards.
Re: Don't forget the comunications and unit structure
Ever since Airfix bestowed the standard 48 piece set on us 50 years ago, I have always wondered why we don't just get a platoon? Most modern armies have about 50 people in a platoon and yet we get set after set where even basic squad weapons and equipment are not in a box of 50 soldiers.
The second box should not be "heavy weapons" it should be the company set with those weapons, radios, officers that are unique to a company. The tripod mounted light machine guns, the 60 or 80mm mortars, the medium range radios.
The third box of figures on a given subject should be the "battalion set" with those items unique to a battalion. The 81 or 120mm mortars, the heavy AA machine guns, and long range radios. And so on up the chain of command for brigade, regiment and division.
Each set higher than the platoon set could have a few extra rifleman poses to make them more desirable so we don't buy 50 platoon sets and one division set, or simply increase the price of each set as they go up the chain of command.
I also wonder why plastic companies don't partner with a metal company to produce figures in the same style that will work together. A plastic platoon set with a metal division commander. Since metal sets can be profitable with fewer sales that seems a natural.