There are as many recipes for every dish as there are cooks.
If the egg white is an addition to the filling I would assume it is to act as a binder that holds the filling together and makes filling the wonton a little easier.
Usually water is used to seal wontons but an egg white could also be used as a sort of glue to help seal the wonton.
I usually read a few recipes and do what seems to make sense to me with the ingredients I happen to have on hand.