Despite being buffed in some areas throughout his SFIV existence, Dan has largely remained a joke character in said series of games. Players commonly use Dan as a way of mocking weaker of less experienced opponents for a variety of reasons. One of the most common being Shotokan players (Sagat is commonly grouped in this section of players) who bypass learning any sort of execution for combos or any mechanic for a projectile throwing zoning playstyle.
Dan's secondary Super Combo could be special canceled into from many attacks. Meaning any attack that could be cancelled can be used as a buffer to Legendary Taunt. Dan players who were trying to mock their opponents would try and end a battle with a cancelable attack and when they got a KO they would cancel the attack into legendary taunt, where Dan would perform the whole comedic sequence and the opponent couldn't do anything to stop it. Later iterations would make this harder as inputs were not accepted after the KO screen appeared. A popular tactic would be to hit confirm Gadoken and input Legendary Taunt quickly.
There are no in-game benefits for doing this. As Dan is meant to enrage opponents in all iterations of the IV games this is more for personal satisfaction as well as making fun of the opponent for losing to such a weak character in such a way.