Oracle tries to do job for you, so if your datatype doesn't match column datatype, it attempts to convert data into correct data type.
Since you have correct xml string in your insert statement, database did conversion to xmltype for you.
And I'm not sure, but I doubt there were a word must use XMLType binding.
But you might want use it, if you need to have more control when creating XML from string.
Check XMLType constructors to get an idea.