The purpose of boardgames is having fun. At least for me. Sure there are people that play them just for the competitive factor. And while that can also be fun at the times, at least when done with moderation I still like the cooperative games more.
Even if everybody is playing towards the same goal it's still nice to feel like winning. Besting the game together feels much better than winning the other players. But it can't feel too easy. Losing together can be as fun too. At least when you don't lose all the time. It keeps up the excitement. It doesn't feel like a win if there is no chance of losing.
Playing cooperative games has another advantage: you can break the rules without hurting anybody's feelings. Sure, those need to be mutually agreed.
But why would you want to break the rules? Especially if you want to retain the fear of possible loss. Well, the games aren't perfect. Sometimes there might be stupid rules. Or rules that contradict in some situations. Playing differently might be more fun in those situations. And as that's the whole point, why not break the rules a bit and have even more fun. Sometimes you might even break the rules to make the winning not so easy.