Why the class NotifyCollectionChangedEventArgs in the ObservableCollection's event CollectionChanged have the properties:

IList NewItems
IList OldItems

As IList (notice the s at the end, also when I use it I have to call it like NewItems[0]) if the ObservableCollection don't allow Add Range, what is the purpose of that if the item modified will always be 1 item ? Or I am missing something ?