Why should we step up the voltage using step-up transformer?

we use step up transformer to increase voltage in secondary side. Now why we try to go through high voltage it is the main point because1) if we keep voltage high then for transfering same powe current means copper loss(i2r) loss will be less.2)According to use efficiency formulae we can see high voltage have greater efficiency.3)If we conveying power in high voltage then area of conductor material is also reduced than low voltage. Now we need to keep in mind we can't increase voltage level beyond critical disruptive voltage because then we have to face corona loss

First you have to Know that the load determines  the power consumption; so the power required fixed for the same load, and so increasing voltage means decrease current ( P=V.I.PF ), and here is the point that u need .. that less current is less money, less voltage drop, which  mean more economical efficiency and more reliable network