The motive of the imposter Moody was to transport Harry to the graveyard and he achieves it with nothing more subtle than a portkey. Then why did he have to help Harry win the whole thing just to make him touch the trophy? Couldn't he just have transformed a book into a portkey and ask Harry to pick it up or anything of that sort?
I believe, although I’m unable to find a reference now, that this was mentioned at some point.
Killing Harry in the Tournament looks like an accident, and helps to keep Voldemort’s return to life a secret.
If Harry had been kidnapped and killed in a classroom setting, then that arouses suspicion. But dying in the Tournament wouldn’t be too surprising: he was underage, and there have been many deaths throughout the years.
Dumbledore would have been suspicious – indeed, he was ever since Harry’s name came out of the Goblet – but the Ministry wouldn’t. Even with an eyewitness to Voldemort’s return, Fudge buries his head in the sand and insists that Cedric’s death was an accident.
If Harry had been killed as well, then the Ministry would write him off as an accident. That gives Voldemort time to operate in secrecy, with the Ministry less alert to signs of his return. (Of course, this happens anyway.) The longer he can keep his return a secret, the more powerful he becomes.