The ballast or condenser acts similarly to a snubber by 'snubbing' out voltage transients during the act of opening the points. These transients occurr due to the points being connected to an inductor (the primary coil circuit). Electrical inductors don't like the current to change so they can jack the voltage way up to compensate. Without the ballast it is possible for this inductor to develop voltages that are too high for the coil primary insulation causing short life or even instant failure.

Kitz