1.) Now most of the games have only 1 type of unit build as the most cost-effective and players always try to use this particular unit build.
It is easy to find the most cost-effective unit build by using the battle calculator.
For example, 2 infantry + 1 artillery is the most cost-effective unit build in terms of PUs.
But if the nations have limited manpower it might be more useful to have 2 tanks instead even if it requires more PUs to spend by helps to save manpower.
It is only example. It could be 3 destroyers instead of 1 battleship or whatever else.
The idea is to make nations to use different unit builds depending on their current PUs'input to manpower ratio.
2.) Now most of the games are unlimited and each major nation can conquer everything.
Moreover each major nation can use PUs' income from conquered territories increasing its force to unlimited size.
The idea is to make nations more limited in the late rounds: a nation with the very aggressive player will run out its original manpower reserve more quickly than the nation with more defensive player.
3.) It is like RAM-memory / HDD-memory
or like fuel consumption / fuel tank
or like production capacity / storage facility
Sometimes this ratio doesn't matter. But in many real life cases it is crucial.
I think the initial manpower limit will make the game more challenging for Axis players both in WW1 and WW2 as they wouldn't have any opportunity to stockpile units and to seat in defence.