Why are filters used whenever a high quality dc voltage is desired?

Question

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Answer 1

DC comes from AC, unless you are using batteries.  AC is, by its nature, very noisy when rectified.  Most devices require some smoothingg out, although devices like lamps, LEDs, motors, etc. can operate just fine on pulsing DC.

Filters take the noisy, pulsing, raw rectified AC and smooth out the rough edges.  The better the filter, the better the DC.  How clean the DC must be is a requirement of the output device.