It makes up for the GAPs in the research tree.
For example, why is it that as soon as we research horseback riding we get a Horse Archer unit?
If we don't have the Wheel, shouldn't we have a Horse rider with a spear?
Sappers are no where to be found in Civ1-4, yet are used throughout history to bring down walls.
Alexander the Great and Rome used Ballistas.
Bombards are basically cannons that ignore walls, normal cannons should have to destroy them first.
Where is the guy with the 2 Handed Sword? That Claymore came from somewhere.
There are no pistol men, only one type of helicopter? Geesh, there is a difference between a Huey used in the 1960s and an Apache attack helicopter.
Machine Guns did evolve. In Civ4, we use the only one created. There is a difference in the Gatling gun used in the civil war, and a WW2 machine Gun, and a Modern 50 Cal LMG.
And FlameThrowers were used.
Now, Repeating CrossBowmen and Compound Bowman were just used to fill in the blanks.
The cavalryPistol could be "CowBoys" for the American Unique Unit.
I forgot to add Towers! Back in Civ 2, I created a speed 0 unit, 2 attack, 3 def, Guard Tower in the Bridge building catagory.
I also had to add Arrows. I modded the version of the cruise missle with to range of 1 with a low attack value.