Trivia: At one point in the movie Shredder scolds Tatsu, and right after that, Tatsu curses saying, "kuso." That's about the equivalent of saying sh*t in Japanese.
Trivia: Famed actor Sam Rockwell ("Iron Man 2", "Moon", "Galaxy Quest") got his start in this film, playing one of the troubled teens seduced by the foot clan. He plays the kid who offers cigarettes to another teen, and delivers the line about the "East warehouse on Lairdman island."
Trivia: Most people know April O'Neil, but as a joke in the movie, two other reporters are also named after months - June and May.
Trivia: Skeet Ulrich, from the original Scream movie, got one of his very first on-screen roles with this movie. He can be seen playing an uncredited thug to the left of the kid who tells the cops to check out the warehouse.
Trivia: Most of the boxes in the warehouse hideout have the "Mirage" logo on them, a nod to Mirage Comics, the publisher of the original TMNT comic book.
Trivia: The four actors playing the Turtles also had cameo appearances in the movie without their costumes: Tilden (Donatello) was the Foot messenger that says "We've been looking for you, Miss O'Neil". Pais (Raphael) is the passenger in the taxi cab that says "What the heck was that?" Sisti (Michelangelo) is the pizza delivery man that can't find the Turtle's address. Foreman (Leonardo) plays a gang member in the warehouse when Casey Jones defeats Tatsu.
Trivia: Although the franchise went by the name "Teenage Mutant Hero Turtles" in the UK, this film retained the international "Ninja Turtles" title, as it helped differentiate it as a little less focused on young child audiences than the cartoon and toy line. Oddly, the film's theme song was still re-recorded to reference "Hero Turtles," perhaps to explicitly tie the film together with the larger franchise and clarify that the "hero" and "ninja" turtles were one and the same.
Answer: Most likely to kill Shredder and make sure that there would be no way to come back but, as seen in the sequel, Shredder does return.