actual need for ballast resistor with electronic ignition?

ballast resistor is all about current control
current flow is dictated by total resistance of the circuit between + volts and ground.
its a series circuit. power supply connected to ballast resistor which is connected to coil which is connected to ground by the points or electronic switch

the relationship is not simple, once the system is in use, the coil's primary resistance will appear to change based on rpm, temperature, dwell, switching speed, etc

but ultimately most ignitions that use a ballast resistor limit the current flowing to 3-4 amps
which is low enough not to burn out the points/pertronix/ignition box but is high enough for that specific coil to do its job across its standard rpm range.
many ignitions that don't use a ballast but do use a 12 volt coil and points just have the resistance as part of the coil, not as a separate ballast resistor unit, these tend to get used on 4 cylinder engines so only have to do half the number of sparks in each cycle, so they can be a bit more leisurely about building a full magnetic field up .

basically your coil and the thing you use to switch it off and on, need to be a matched pair,
and a ballast resistor may or may not be used. the need is dictated by the specifics of the other 2 parts.

Dave