The role was carved out for government backbenchers purely for Nationalist Party reasons, not to satisfy any constitutional and public need, as in the case of Cabinet ministers and parliamentary secretaries, who head the Executive according to the democratic separation of powers.
An opposition leader deals with the Prime Minister, not with component parts of the executive. Were opposition members to go directly to members of the public administration with their queries, the government would surely yell 'foul!' And rightly so.