A good manager will know that s/he will be given credit for good work. Letting the team (especially junior members) lets them take ownership and pride in their work. And some nice words from the senior level present in the room always goes down well, which keeps the team happy.

I don't think it's a fair to compare an internal demo of a team's work to colleagues with an external demo of a company's products to journalists and the public. To take your reasoning: marketing and sales should also be done by engineers, which is not the case.