It might be used to short out the resistor once the lamp warms up.

It short-circuits the auxiliary electrode to the main electrode, and has been a feature of all probe-started metal halide lamps since the 1970s. In case the two adjacent electrodes are not short-circuited and the arc tube filling contains sodium, the small voltage difference between them (caused by the probe tip just touching the edge of the plasma) will cause electrolysis of the quartz. That accelerates the rate of devitrification, and leads to weakend quartz and ultimately explosion of the arc tube. This was a serious problem which limited the life of the early metal halide lamps. But by short-circuiting the two electrodes to keep them at the same electrical potential this problem is overcome, and lamp life is increased dramatically.