Why is water added to the egg wash of challah bread? This recipe for challah bread calls for an egg wash composed of 1 egg and 1 tsp of water (see the final item in the ingredient list).

What is the effect of this small amount of water?  It seems like most other challah recipes use egg alone.
If anything, for egg washes in general, it might be more common to add water (or milk or cream) than not.

Thinning the wash makes it easier to brush on an even, not-too-thick layer. It also tends to get you a more golden color, less dark brown.