My guess is
That it gives the Office of National Statistics a reason to validate its budget as much as anything else- particularly when it comes to voluntary activities. I can see the point as regards employment as a way of measuring the effects of equal employment legislation, however I think there is a misconception about that too; namely that equal employment legislation merely states that it is illegal to discriminate against people on certain bases and does not actually set quotas which is the way it's often seen.
( Lord Gnome, Thu 28 Aug 2008, 13:46, archived)