Tony the Tiger's catchphrase is "They're grrrrrreat!" because he thinks that Frosted Flakes are great and he's a tiger and tigers say "grrr."
"Grrr" is the sound of a tiger growling. Growling is a tiger's signal for feeling pissed off or threatened. But Tony sounds pretty jovial and upbeat whenever he's advertising Frosted Flakes. So why is he growling? Is he pissed off? Does he feel threatened by Frosted Flakes' competitors? It doesn't appear so. If he likes Frosted Flakes so much, he should be purring. A more realistic slogan would be, "They're prrrretty good."
Secondly, with Tony growling all the time, it gives the impression that he is first and foremost a tiger and only a breakfast cereal spokesman second. But if this is the case, then his priorities are inconsistent. His first priority seems to be food, which is consistent with that of a tiger. But he doesn't seem to be as concerned with eating Frosted Flakes himself as he is with getting other people to eat them. If Tony were first and foremost a tiger, then he wouldn't care if anyone else was getting food.
Some may argue that this is the sole reason why Tony, and not some normal tiger, was chosen as the spokesman for Frosted Flakes. However, look at other breakfast cereal spokesmen that are animals. You don't have to be concerned with satisfying anyone else's taste buds to be a cereal spokesman. Toucan Sam doesn't care if anyone eats Froot Loops. He just wants a good life for himself and his three nephews. And The Trix Rabbit definitely doesn't care if anybody else eats Trix, even going so far as to steal it from children. And that crazy Cocoa Puffs cuckoo bird, Sonny, is so self-absorbed and whacked out of his mind he probably doesn't even realize there are other people on the planet.
If Tony were like one of these spokesmen, an animal just trying to eat, then everything would make sense. His commercials would feature him just eating Frosted Flakes by himself and exclaiming that "They're grrreat!" He wouldn't be telling this to anyone in particular. He would just be talking to himself and expressing his emotions aloud. It would be as if a camera just happened to be filming it. We would be watching a tiger eat his favorite food. Hearing an occasional growl would be absolutely believable.
But those aren't the commercials. Tony is a tiger happily promoting Frosted Flakes and growling at the same time. It makes no sense.
He's also talking and tigers can't talk, but that's not that big of a deal.