Even the wealthy can't escape their craving for attention, and charity is their most "cost-effective" tool. After all, a donation leads to endless media exposure, and their public image skyrockets. On the other hand, it is precisely because of these celebrity stunts that the public’s focus is drawn to distant social issues.
There will be a portion of celebrities that do it for their ego or because a public relations person has told them to improve their image. However there are plenty of them who do it for purely altruistic reasons and probably do more for charity than the contributions of millions of others collectively. It shouldn't really matter to you anyway, as a single extra dollar is going to good use whether it's for vain reasons or not. I think it's a great thing how people like Bill Gates are pledging to donate billions after they die and wish others would copy that model - even better when they start early. Media exposure for the people at the top is probably the last reason, because they've already had more than enough.