This research doesn't answers why we brag. It just tells us what we already know.. that it feels good to talk about ourselves, but why?
"Should I hunt a deer? I don't know, I don't think I'll ever know." Then dies of starvation. At some point you need that little drop of arrogance to tell you. "That's it, I'm right, I need to do this." Even tho, technically, you can't be sure.
So a little bit of arrogance is necessary for animals to function. We evolved into beings with a drop of arrogance because the ones without this trait died off. Natural selection made sure both extremely humble and extremely arrogant personalities were evolved away from.
And bragging about is just one of the many consequences of arrogance. We're arrogant + we can communicate = we brag.
On the other hand, it’s also great to talk with someone who’s knowledgeable about your interests. That seems to have less to do with pleasing oneself by speaking as an authority, and more to do with sharing and collaboration. My major is not in computing, so it was a breath of fresh air to discover CS professors and graduate students with whom I could speak freely.