The usefulness of transformers
A transformer transforms potential difference. In particular it can step up or step down the potential difference by a constant multiplier.
But you don't get something for nothing. Energy is still conserved, so the power in the inbound (electrical) pathway is nearly exactly equal to the power in the outbound (electrical) pathway. The transformer doesn't accumulate energy, having no stores of energy associated with it – the transformer is just a device. In fact the engineering can be so good that nearly all of the power stays in the electrical pathway – very little gets diverted to different pathways (less than 1 percent for large transformers).
Different characters of pathway
However, the character of the electrical pathway can be altered – you can switch a small potential difference, large-current pathway to a large potential difference, small-current pathway. The same power is carried in the pathway, but there are advantages to pathways of particular character, which were explored earlier in discussions about the choice of 230 volt as the domestic supply.
The device plays a crucial role in any large-scale electrical supply system, where its role is to alter the character of the pathways to reduce losses by reducing switching to other pathways in the transmission wires.
Households contain many transformers, because small electronic devices – which are very common as a part of larger appliances (do you have an LCD display on the washing machine, dishwasher, or oven?) or on their own (the computer, alarm clock, electric shaver, or music player) – need small potential differences to work and don't work well on 230 volt. If 230 volt were used, the resistances would have to be huge to prevent large currents where they weren't needed, and so the compact and handy devices that we have now wouldn't have been developed.
However, not everything that appears in a small black cube on the end of a socket must contain a transformer – there are electronic devices that can now bypass this process but will still change the character of the pathway, so care is needed.
In systems for shifting energy across large distances, there will typically also be many transformers. The cost of running electrical power lines at 400 kV (much of it associated with mitigating danger to people) is only justified if there are great distances to be covered and if the power in the pathway is huge (e.g. linking a large power station to large conurbations). For local distribution, say across a town, power lines can be at a smaller potential difference, perhaps 11 kV, so the pylons and other devices designed to keep people from being part of a circuit containing such a large potential difference can be smaller, with smaller capital costs as a consequence. Remember that we are comparatively safe – particularly with dry skin – with a potential difference of 230 volt across us. However, factories and other industries may be supplied with much larger potential differences and, as a result, be subject to much more careful safety procedures.
As there will be several different compromises, depending on the distances from generating stations, and the power demands and safety needs of end users, there will need to be many transformations of potential difference, each likely to have its own transformer.
Some high-voltage systems do use direct current, but these have to rely on devices other than the transformer to change the character of the pathway.