Transformers for Powerlines - Practical classroom demonstration

Why do we step up electricity supplies only to step them back down again in National Grid systems? This video demonstrates the need for transformers to step up and step down electricity supplies to enable more efficient distribution of electricity over long distances through power lines.
The demonstration simulates a power station providing electricity to a domestic user wanting to light a light bulb via some long and thin power lines. Simply applying the low p.d. required by the lamp across the power lines fails to light the bulb. But the subsequent use of transformers to step up the supply prior to transport across the lines and to step down the supply prior to use by the bulb enables us to overcome that problem.
Relevant concepts: electromagnetic induction, transformer p.d. ratios, power loss in wires, P = I² R, efficiency.

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