Forgive my ignorance, A/M ignition

[sv51macross]

A thread about a MSD ignition module on RX7club got me thinking; I have little understanding of how these units work and how they are integrated with the engine.
ECU sends signal to coil, coil fires, the uber-high voltage goes through the plug wires and/or directly into the plug where the voltage sparks across the gap and grounds through the block.
But where do these ignition modules figure in, and what do they do? I take it they aren't coils themselves, but controllers? And a faq on AEM's site claims that their twin-fire module was designed with rotaries in mind...?
Might someone please educate me on the purpose of these A/M modules? What do they do that upgrading the coils/wires/plugs [and ensuring proper connections/grounds] doesn't?