Legendary Magician of Dark says that if it attacks reduces the opponent's life points to 0 and some other conditions are met, you win the match. But wouldn't you win anyway just by having reduced their LP to zero? What's the point?
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 :)
You win a duel by reducing your opponents life to 0 and you win the match by winning 2 duels. So what this means is you can win the match in 1 duel instead of 2.
The first Duelist to win two Duels is declared the winner of the Match. If a Duelist loses two Duels, they are considered to have lost the Match. If both Duelists have one loss in a Match and both receive a simultaneous Game Loss penalty, the Match is considered a Double Loss
Reducing your opponents life points to 0 is one of the ways that wins you a duel, a match is the best 2 out of 3 duels. The Match Winners cause you to win the entire match in one duel, once their conditions are met.
The first match winner was Victory Dragon, sent out as a promotional card in Shonen Jump magazine. When it came into play, right before you would lose the duel to it's effect people would simply concede the duel, to have a chance to win duels 2 and 3 and still win the match. This caused Victory Dragon to be forbidden very quickly for tournament play.
After Victory Dragon, all match winners have been prizes at high level tournaments, and all match winners have the same forbidden status, they are more showpieces than usable cards. In fact, after that first one, they all have forbidden status printed on the card, just below the text box on the left: "This card cannot be used in a duel."