Super Smash Bros. has changed a lot since Melee arrived on the GameCube in 2001. However, even after all that time, fans remain loyal to the game, citing design flaws seen in future games such as slower gameplay and lack of advanced techniques.
One of these flaws in particular that gave many Melee loyalists nightmares was the tripping mechanic introduced in Brawl. The mechanic, which is not present in Super Smash Bros. for the Wii U/3DS, did exactly what it sounds like: make characters trip and fall straight on their faces in the middle of a fight.
Now, Melee fans get to relieve that nightmare, thanks to Dan Salvato, creator of the Melee mod 20XXTE, who created a mod called Project B, which allows characters to trip in Melee. It isn't a real mod, as it won't be released (or at least there are no indications that it will), but just by making the mod, he has shown that he can easily incorporate tripping into 20XXTE, and no one would be any the wiser until the inevitable happened.
With that said, why bother making a mod like this at all? You would have to ask Salvato himself, but one simple answer would be that it's downright funny to see it in action. However, that humor goes away really fast in the middle of a match (hence why he didn't release the mod). While characters had only 1/100th of a chance to trip every time they went from a still position into a dash or roll on normal ground (the chances increase on ice or wet surfaces), that small chance could mean the difference between victory and defeat, because it leaves them vulnerable to attack for half a second.
It was one thing in Brawl when tripping wasn't too costly due to the slower nature of the game, but due to the nature of Melee, tripping could easily lead to losing a stock. Of course, tripping was only ever present in Brawl, but it (along with other gameplay features) ensured that many Melee fans would remain attached to their game of choice and never move beyond it.
As for mods that will actually be making it to Melee, Salvato is working on adding an airdodge-canceling mechanic. Since Smash isn't a traditional fighting game, it's unlikely the next official game would have such a mechanic, but it would be cool if it did.