Why is baking soda included in sourdough cracker recipe?
In addition to leavening, baking soda increases the PH of the dough. Since your recipe uses sourdough starter, it may be fairly acidic. Adding baking soda will make the dough less acidic. The PH level, in turn, is important for browning: a high PH level facilitates the Maillard reaction, giving you brown and toasty crackers.