Why is high voltage needed to establish a spark across the gap of a spark plug?

High voltage is needed to establish a spark across the gap of a spark plug because it creates a strong electric field that can ionize the air in the gap. This ionization reduces the dielectric strength of the air, allowing current to flow and forming a conductive path for the spark. The high voltage ensures that the energy is sufficient to overcome the resistance of the gap and create a reliable ignition source for the fuel-air mixture in an engine.