Basically the same thing you stated here to bring you closer together.  The 2 other guys that I pledged with are probably some of the closest friends I have because they were there with me enduring the hell that we went through, both being physically and mentally broken down throughout the process.  It's basically like practice, just a practice to teach you how to manage your time, work and respect those above you.  It teaches you seniority (AKA a hierarchy), basically like respecting your boss at work, or your parents. And in all honesty it was some of the most fun I have ever had and taught me a lot about myself, and how to be a better man.
I'm not saying anyone is wrong for taking the side against hazing, and I'm not saying all hazing is right cause some of the older traditions my fraternity had were fucked up beyond belief, but some people over react to some shit that won't kill you but breaks you down and then builds you back up.