The constructor referenced by this property is that of the built-in Boolean prototype object.
You can use this referenced constructor as one way of creating Boolean objects, although it is more popular to use the new Boolean() technique. This property is especially useful if you have an object that you want to clone, but you don't know what sort of object it is. Simply use the property to access the constructor belonging to the object you have a reference to.
Netscape provides constructors for many objects, virtually all of them in fact, even when it is highly inappropriate to do so. MSIE is far more selective, so there are some occasions when you might wish for a constructor that MSIE does not make available.
ECMA 262 edition 2 - section - 15.6.1
ECMA 262 edition 2 - section - 15.6.2
ECMA 262 edition 2 - section - 15.6.3
ECMA 262 edition 3 - section - 15.6.2