They boost and control spark output over a wide rpm range.  You still use a coil with one of these boxes.

Steve

They cannot actually boost the spark, but they do modify it and make it more efficient.   Instead of one long spark it creates a short spark, stops the spark, and creates another short spark.   This can be repeated 4 or 5 times at low rpm, and decreased the number of times as rpm increases.  This is due to less time for optimum spark. The advantage is having a much longer total spark time.  This allow more fuel to be lit, and make a more efficient burn.  But the coil cannot do more than the coil can do in terms of max output.

The MSD is a very good upgrade, even with a stock ignition.   If you really want to give it a kick, use a matching coil, cap, rotor, and plug wires.  You can get a lot out of this combo.  

DON'T install a High performance coil without the wires, cap, and rotor.   That will overload the ignition system and cause more problems than you can believe.   But the MSD control box can be used alone with stock ignition, and help the engine.

I also used the malloy with a rev limiter...noticed the difference especially at low speeds.

Alan

DON'T install a High performance coil without the wires, cap, and rotor.   That will overload the ignition system and cause more problems than you can believe. 

YES , Ken , I believe.