Without the exemption in place:
- If you do a "dive-and-drive", you're allowed to descend to the MDA, have a look around, and stay there until the MAP in hopes of seeing the runway environment.
- If you fly CDFA, then you're only allowed to descend to the DDA (MDA + 50ft) before a mandatory missed approach at that point, even if you haven't reached the MAP.
This gives a "disincentive" from flying CDFA. You are essentially being penalized both in terms of minimum altitude and missed approach position for "doing the right thing."
Of course, some ICAO states simply say "tough luck, if you're an air carrier we're going to require you to fly CDFA until a DDA, by regulations".
TC and FAA have taken a different approach by removing the disincentive.