OK, interesting info. If you had asked me I would have said that its purpose is to ensure the engine gets a good spark during crank.

When cranking you are relying on battery power alone and the starter is using a lot of current (taxing your battery). The engine is cold and turning at low rpm. You need a good spark, so a full 12V is applied to the coil. It's designed so that in normal usage (running, hot) it does not require that voltage, in fact it would wear it prematurely, but it does need some extra zip for starting. The solution - provide 12V during crank and a reduced voltage once running - this is the purpose of the ballast resistor.

I didn't know the other stuff about varying the resistance with heat (yes I know that can be a function of a resistor but didn't think about the automotive ballast). Makes sense, but I think the primary benefit is the 12V start/reduced V running, and suspect that any variation in the (reduced) running voltage is really just further refinement to save more wear and tear