So lets add a capacitor to the points…usually on a DC circuit. To keep the spark from welding the points together one can use a cap across the points. As the points start to open the potential of one contact will fall and an arc will form jumping between the points. A capacitor will discharge into the lower potential contact keeping the potential equivalent until enough space opens up between the contacts to prevent arcing. When the cap is fully discharged the circuit is open.

On a spark system the cap keeps the charge in the coil from leaking back out through the arc of the points, loosing coil energy. The points open, cap keeps the coil charged until enough space develops between the points. When the cap is discharged the mag flux collapses into the secondary coil causing the plug to spark. If too much cap..the arc will move material to one contact…if too little the other contact will gain material.

..ya didn’t ask about this but..