Which are .... what, exactly?

Not to belabour the point (which I am, I know...) but I'm still struggling to understand the logic of QF's approach.

I fully understand the notion of buying a 'cheap' ticket that has -
(a)an advance purchase requirement
(b)is nonrefundable
(c)requires a change fee if you change it
(d)requires you to pay the fare difference between what you originally paid and whatever fare is now available.

To this QF has now added -
(e) and if you change it or cancel it on the day of flight, we won't give you any credit for it (though if you do it before the day of flight, we will).

This clearly just adds one more way to make the customer pay (which is fair enough). However, what's the logic of allowing changes prior to the day of departure but not on the day of departure? Why not simply say "no refund, no changes, no credit - no matter when you change". How does QF benefit from not allowing a change on the day of departure that would be allowed if made the day before?