Prepares you for the real world. People complain that schools don't prepare you for adult life nowadays; Marianopolis isn't one of them.
An analogy for the competitiveness here is like playing hockey in the NHL. It's obviously more difficult to play in the major leagues than at your local rink; you're surrounded by the best. For those who aspire to be great, this is the biggest perk. When you make it here, you know you can make it anywhere; when you're not good enough at first, you can learn from the best to improve yourself so that when you leave here, you'll be NHL material.
Going to an easier CEGEP will only shield you from this competitiveness temporarily. When you reach university, and later in life when you're competing for jobs, there will be Marianopolis alumni also vying for what you want, and you'll have to compete with them eventually. When I attended, I learned so many tactics people employed to gain leadership positions in clubs, to secure interviews and offers, to study efficiently, and to achieve their dreams. I forgot most of what I learned in the classrooms now, but all those soft skills remain with me today. When I look at the LinkedIns of my former classmates, I can't help but be impressed at the careers we're all leading today: surgeons, investment bankers, management consultants, big law, M7 MBA, software engineers at FANG and in quant finance. Even the ones who graduated with an unimpressive R-score eventually achieved great things.
Come to Marianopolis if you want to experience what being an adult is really like for the first time in your life (red pill), or go to a party CEGEP if you wish to just continue being a teenager 2.0 (blue pill).