When a current passes through a cable it has a heating effect, so some electrical energy is wasted as heat. The bigger the current, the more energy is wasted. The national grid is designed to keep this loss small.
The power transferred is the potential difference multiplied by the current (power = p.d. × current). For a given power, a higher voltage means a smaller current. For example, 1000 W at 100 V needs 10 A, but at 400 V it needs only 2.5 A, so a p.d. 4 times bigger gives a current 4 times smaller. Step-up transformers raise the voltage for the long-distance cables, so the current is small.
A smaller current means less heating in the cables, so less energy is wasted and more reaches homes and businesses. The grid is therefore more efficient. Step-down transformers then make the voltage safe for use in each locality.