With Treat Training alone, you must give the treat to your dog while they are in position. For example, to reward a sit with Treat Training alone, the dog has to get the treat while in a sit. If he stands up and THEN gets the treat, he was rewarded for standing - not sitting. To be fast enough, you may need to hold the treat which can result in your dog only working if he sees you holding a treat.
The "right answer" cue fixes those problems because you don't have to give the treat to the dog while they are in position. You only have to give the "right answer" cue while they are in position. Then, give them their paycheck. The treats stay in your bag until earned, so Fido doesn't become treat dependent.