Whiffing a move in the air cuts your landing recovery. You can do them as a fake, with the easiest example being do a jump c on the way down from a jump, but make it whiff. This makes your opponent think that a jump attack is coming, then you land and can either grab or go low, to beat the high block. In this example you can instead do a jump c that hits and you get yourself a mixup. The idea here is that it is just a distraction, and a lot of things can be done with that idea. Some moves have mobility on them, and you can use them purely for that instead of the attack part. In some games you can get meter by whiffing moves. It can be a timing mechanism, such as if after so and so, if you whiff 5b then do 5c right away it hits meaty, which may be easier to time. You can use moves just to fill space and stop your opponent from getting into that space.
The canceling of Cs into As isn't to whiff an A, it's to cut recovery and get more frame advantage. The A whiffing isn't the goal, much like in the first example I gave. The whiff jump attack isn't done to whiff an attack, it's done to remove recovery. You can do the same thing with dashes. The same can be said for some of the examples I gave, it's just these types of things are character/game specific and take advantage of specific system mechanics where it isn't the whiffed attack that you want, it's, in most cases, faster recovery.