but trying to add and subtract all those numbers for each unit while you are busy giving orders and watching your enemy
You're right. Unless one is some sort of math savant, it can't be done. But knowing what math the game engine is applying on the battlefield can help with the decision-making; like what to expect if your general gets killed, or which units will withstand gunfire better, or which units are likely to rally from a routing condition, etc.

The obvious ones an experienced player will know (maybe not the actual numbers, but the expected results) ie: you get flanked or attacked from behind, the radius of effect(s) from your general, fatigue, taking persistent missile fire, etc.....