It's a hangover from when the committee stage of any money bill went to the "Ways and Means select committee". The 1st Deputy Speaker is also the Chair of the Ways and Means committee (even though it no longer exists) and so he/she would always chair the parliamentary reading or any money or procedural debate - this was because there was always a suspicion that the Speaker was an agent of the Sovereign and would use this power to unduly influence the allegedly democratic mandate of the Members.
NALOPKT(&EFGAS),
PDR