Not exactly necessary, but certainly beneficial for the HT circuit.
Has to do with how this ignition system works. First some understanding of the circuit needs explanation.
When the points close (or other electronic trigger device) current flows through the coil's primary and a magnetic field builds in and around the metal core material of the coil. Ideally, this time duration is enough to achieve full magnetic field saturation. This is where power is put into the coil.
The secondary coil circuit path is from one HT wire through the spark plug, across the cylinder head and through the other spark and HT lead. The circuit need only be completed by a conductor across the spark gap (plasma arc). The secondary windings in the coil (where the HT leads are attached) are physically located within the same magnetic field created by the primary current. When the points are opened, the coil magnetic field collapses, and the collapsing field creates a voltage in the secondary windings. This voltage rises until there is enough voltage to create a plasma tunnel between the electrodes of the spark plug(s). This also completes the secondary electrical circuit so current can flow in the secondary of the coil. If the HT circuit has low/no resistance, the energy dump from the coil is a very rapid, short event, with the highest peak current the system is capable of producing, limited only by the secondary resistance of the coil itself. The high currents promote erosion of the spark electrodes. (see also ESD machining, spark gap erosion machining).
If we add resistance to the HT leads, the voltage needed to create the plasma channel and fire the spark plugs is exactly the same, but the in-line resistor limits the current during the spark event. It also extends the spark event because the energy dump from the coil is slowed down. This gives a bit more time to ignite the mixture in the cylinder. It also extends spark plug life by reducing the amount of erosion that occurs at the spark plug electrodes due to reduced current peaks.
So, HT resistance, rather than being detrimental to spark, it is actually beneficial.
But, you can get too much of a good thing, though. The current passing through the spark arc, does relate to how hot the plasma channel becomes during the arc event. If you use too much HT resistance, the plasma channel heat can get too low, and effect the sustained duration of the spark event, and it's ability to effectively light off the air/fuel mixture. That is why, you normally pick one of the resistance devices found in the HT circuit; Resistance wire, resistor plug caps, or resistor plugs.
