You should learn more about the onchange decorator odoo. deletion and creation are not promoted here, it is dealing with virtual data and after the process, it is sent back to the client. If it is not returned back the data will not get saved.

This is the definition provided by odoo

""" Return a decorator to decorate an onchange method for given fields.
    Each argument must be a field name::
        @api.onchange('partner_id')
        def _onchange_partner(self):
            self.message = "Dear %s" % (self.partner_id.name or "")
    In the form views where the field appears, the method will be called
    when one of the given fields is modified. The method is invoked on a
    pseudo-record that contains the values present in the form. Field
    assignments on that record are automatically sent back to the client.
    The method may return a dictionary for changing field domains and pop up
    a warning message, like in the old API::
        return {            
            'domain': {'other_id': [('partner_id', '=', partner_id)]},
            'warning': {'title': "Warning", 'message': "What is this?"},
        }
    .. warning::        ``@onchange`` only supports simple field names, dotted names
        (fields of relational fields e.g. ``partner_id.tz``) are not
        supported and will be ignored"""

https://www.odoo.com/documentation/11.0/reference/orm.html#module-odoo.api