Celebrities don't necessarily have to announce their donations to charity but doing so raises awareness in the public eye and encourages people who are not celebrities to also give, even if it's not as much as a celebrity would donate. Every little bit counts and awareness helps charities as well.
There is no reason a celebrity shouldn't be allowed to announce their charitable donations. Sure, sometimes it just seems like bragging. However, there are plenty of times when a celebrity can raise awareness by voicing their support to a certain charitable organization. Many celebrities have great influence over people, and can use this as a way to get others to give as well.
While it is good for celebrities to be proud of their contributions to society, it would be better if they remained humble and didn't boast about it. If they announced their donations, it would seem like they were doing it for praise as opposed to doing it for a good cause.
This is awesome to hear. One thing I think he has that other celebrities don't is that he's not always instantly recognizable.
With actors in film and TV, you hear them talk, see them move. You run into them in public and most of the time right away KNOW it's them. You've seen them enough that the subtleties of their very movement immediately triggers recognition.