The output of the voltage multiplier will deliver less current than the circuit driving it. If your multiplier can draw 30mA on the input side then it will never deliver 30mA on the output.
Voltage multipliers are used to generate high voltages from lower voltages - duh! You use that kind of thing when you need high DC voltages. The ouput of a voltage multiplier is DC even though it starts with AC or pulsating DC.
Wikipedia gives an explanation of how they work (with diagrams) and also names some uses - extremely high voltages for physics experiments being one use.
Another common use is (was) to generate the high voltage for cathode ray tubes - that's the old fashioned TV tubes in monitors and TVs. They needed several thousand volts DC to operate.
A good reason to use a multiplier is that it can be difficult to rectify high AC voltages. That is to say, you could use a transformer to get the needed high voltage, but then you'd have to convert the high AC voltage to DC. High voltage parts are more expensive and/or harder to get. Using a multiplier lets you use parts rated for lower voltage to generate the high voltage. Your parts only have to be rated for the difference in voltage between each stage of the multiplier rather than for the whole voltage range from low to high.
In answer to the questions in your comment:
- It is a bad idea to drive anything directly with the output of the processor's output. Use the output from the processor to control a transistor that drives the multiplier. This protects the processor and allows you to have a higher output current.
- Frequency limits wil be given by the diodes and the capacitors you use. Diodes all have an upper limit to how fast they can switch. The capacitors can only charge as fast as the available voltage can make them, so you are limited there as well.
Do read the Wikipedia article and the information in the external links.