In professional sports it can be. Your teammates can feed off a your confidence. It's the mental edge. If your arrogance can affect some of the weaker minded players on the other team, you've already scored an advantage.
Thats probably about the only positive thing that you could get out of it, really. in the vast majority of times, it is done with a fair dose of insult to other players, competitors and whatnot IMO. Never seen a such thing as clean, innocuous brag without putting down others in the process deliberately or subtly.
I think our upbringing has a lot to do with it, seeing how most cultures value humility as a fine virtue to be pursued, though to a varying degree. To me, act of pompous self-promoting or display of inflated ego comes across as the person being either selfish and self-serving, or just trying to cover up his weak mentality.