What is the point of Legendary Magician of Dark's effect?
The key difference here is that a duel consists of a single game and also a match consists of a best-of-three duels game. Official Tournaments consist of rounds of matches.

So, with this card if you manage to reduce your opponent's LP to 0 and fulfill the other conditions you win the best-of-three games immediately.

Be aware that Legendary Magician of Dark is one of the Tournament Prize Cards that are awarded to World Tournament Champions. All these have the common trait of making you win the match.

Unfortunately, this cards cannot be used in a oficial tournament; you can only use them on friendly and traditional games, that is, if you have won one of course :)