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A transformer has no role in rectification; rectifier is a device that allows current to flow predominantly in one direction and is useful in converting AC into DC.
I mean in the input side of rectifier circuit, we use a transformer.
Why that transformer is used?
Books say, to provide isolation, and to control the output DC voltage of the rectifier.
Can't we use a DC-DC converter at the output end of the rectifier instead of transformer to control the output DC voltage?
1. to provide isolation
2. to drop the voltage to the desired amount(s).
You can use a DC-DC converter but what is the point. For example, if you have 220V AC available and you need 12V, you can drop it in a transformer, rectify it and use or regulate it if necessary.
If you didn't use a transformer, you rectify it to about 312V then have to find a way of losing 300V at your load current and you have no isolation either.
In simple terms, the transformer lets you trade Volts for Amps, in most cases you want low voltage out at high current and have a high voltage available from the mains. The transformer converts it in one cheap component.
I am not saying the classic transformer/rectifier is optimal in every situation but in most cases it is the most economical and simplest component to use.
The single most important role of a transformer is level conversion. It is an easy and effective way to transform AC voltages to different levels at high efficiency.
When power is generated, it is often produced at a "low voltage", say 33kV or even 10kV. For transmission, this voltage us stepped up, say 110kV or even higher. Somewhere close to the consumer, the voltage is reduced to 220V (or even 110V) and fed to homes. All these are done at high efficiency, perhaps close to 96-99% power efficiency (at individual transformers). So when you will need 6V or 12V (to feed your small gadgets), a small transformer is again called for.
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