First of all, you want the light to go on, not off.
The idea is that the spark is supposed to happen when the points open.
When the points open, then the current is no longer being grounded away through them, and then you will again get power on the negative of the coil.
In theory the coil could fire when points both open and close.
But you can't operate the coil from when the points close, because it takes too long for the coil to build up voltage. But is can quickly discharge the voltage when the points open.
The purpose of the condenser capacitor, is to essentially eliminate the closing spark completely. So you only get a coil discharge when the points open, removing the ground connection.
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