Although it is not compulsory, members of the House of Representatives begin their speeches by saying, ‘Thank you Speaker’.
Members do this as a reminder that speeches, questions and comments are to be directed through the Speaker of the House of Represenatives. One of the Standing Orders—rules—of the House states, ‘Members addressing the House do so through the Speaker.’ This helps ensure the orderly running of the meeting and discourages members from engaging in private argument.
It is also an acknowledgement of the very important role the Speaker performs in the House. They are in charge of the meeting; it shows respect for their position.
Finally, it is a polite thing to do. It’s nice to start a speech by thanking the person who called you to speak.