Assume that you have

class ClassA

    lines: fields.One2many('class.b', ....)

and class ClassB

     def action_do_something(self):

          ...

Now in the form view of class A, you add a new item. What happens? you see a new line of class B in the list but it is only in the browser, it does not exist on server yet. Your button is linked to action_do_something of a not yet exist object of class B. How can you call that function of a not exist object? You can't. You must save object of class A first, so that new object of class B get saved too (actually, it is created). Then you can call its method.