Having both options ensures you see the film as Noé intended—whether that is the nauseating rotating camera of the Rectum scene or the serene, steady shots of the final park scene.
Below is an overview of the film, its unique structure, and the technical versions you might encounter. Film Overview: A Descent into Chaos Released in 2002, Irréversible Irreversible -2002- Dual 1080p
I am not going to describe the 9-minute centerpiece shot. If you know the film, you know exactly which scene I mean. Having both options ensures you see the film