how to convert ac to dc 5v 9v 12v

how to convert ac to dc 5v 9v 12v AC & DC converters is electrical circuits that transform alternating current AC input. into direct current DC output. AC to DC converters use rectifiers to turn AC input into DC output regulators to adjust the voltage level. and reservoir capacitors to smooth the pulsating DC. why do we convert AC to DC we need to convert AC to DC because of below facts AC signals can not be stored and DC power or signals can be stored.
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