The office of Speaker is almost as ancient as Parliament itself. It emerged in the Middle Ages when the Commons - the ordinary people - of England needed a spokesman in their dealings with the King, someone who would voice their grievances and present their petitions. This was by no means a safe or easy thing to do at that time, and potential spokesman generally had to be pressured into accepting the responsibility.
Chapter 4, The Office of Speaker of the House of Commons, p. 46 - The House Of Commons At Work (1993)