The point is to give teeth to enforcement of international smuggling laws. If you saw the ploughshare news today (54 ploughshares confiscated at Bangkok airport), you get the point. At the least, CITES I species are sold less openly and less frequently than they would be if they international commerce were not banned. I agree that CITES I designation sometimes does more harm than good, but it's a case by case basis.

Don't forget, there is very little evidence that captive breeding can actually save a species, except in captivity, no matter how much we all want to think we're doing something moral by breeding rare tortoises.