Why should I rest gingerbread dough?
In most wheat-based recipes, gluten strands are formed while you work the dough. If you don't rest the dough, rolling can be more difficult and you may get a tougher texture in the final product.

Additionally, refrigerating the dough up to a certain point will firm up the fats in the recipe (butter, etc), which will make the dough less mushy and will making rolling out the dough evenly a bit easier. Unless the dough is so firm it becomes brittle, refrigeration will make the dough less fragile in most cases.

Perhaps less important, the moisture levels may become a bit more consistent throughout the dough as well.