It sounds so insignificant, so silly, but it is the natural way to build camaraderie between team member. People lower their guard, and have nothing to 'prove', unlike...uh, work.
This strongly depends on the type of people you are working with. If I knew I'd be expected to have group lunches every day I would not even bother applying to the company. Based on my experience with team lunches, it means that I can expect at least an hour(often even more) of every day to be lost to uninteresting, forced conversations. I am so much more productive now that I have a job where I can casually continue my work while eating lunch alone.