There are so many reasons! I’ll pick a few big ones.
First of all, you know when I said earlier that without having any kind of support, making a story isn’t going to be too much fun? Here’s the thing: having other people making the story with you is the best kind of support you can have. Feel like you’ve hit a brick wall? Ask around; someone will be packing that sledgehammer.
Closely related to this is surprise. Many wonderful stories reward us because things happen that we didn’t expect. The plot twist that turns everything that came before upside down, the character who betrays our expectations and transforms our attitude towards them. (Great stuff.)
Now, I guarantee that when you make a story, you will surprise yourself. Guarantee it. Things emerge that you didn’t plan at all, but when they strike you they make perfect sense and make you shiver. (In my opinion, this is even greater stuff.) By making the story with other people, you are ensuring you will hit these unexpected moments far more regularly. And crucially, they wil l shake things up where you would never dare: the aspects of the story that you had considered inviolate. And your blindspot might just be the sweetspot where truly excellent story lies.
Finally, the fun factor. Any activity done socially has the potential to be a lot more fun. The more you like the people involved, the more likely you are to get this extra fun. This fun is distinct from what you get from the content of the activity (in our case, the making of a story) – it’s the fun you get at the level of relying on people, engaging in competition, and learning something about what makes the other person tick. It lies behind team games, going hiking in a group rather than solo, and having a coffee break. I know, this is basic stuff, but it’s always worth putting out there. Doing stuff together can enhance any activity, and in an atomised world, it’s great to do.