Adding boiling water to flour causes the starch granules to swell and gelatinise, allowing the dough to absorb more water, resulting in a softer and/or fluffier finished product. However, a dough made entirely with boiling water lacks extensibility (i.e. can't be stretched) because some of the precursor proteins to gluten are denatured at such a high temperature. This may be desirable as less gluten means the dough is easier to roll out thin.
On the other hand, dough made entirely with cold water has high extensibility, especially when gluten is allowed to develop through time and kneading, giving the finished product more chewiness.
Your recipe uses a combination of both types of dough, balancing between both extremes so that the finished product is not too soft but not too chewy.