It allows one to disavow any sense of social reciprocity after becoming obscenely rich.
I was curious, so I looked through his Wikipedia page -- it says he donated $1m to the Hebrew Immigrant Aid Society in 2009 (which helped his family move to USA when he was a child). Even the NYT article notes that "The gift is small, given Mr. Brin’s estimated $16 billion in personal wealth" :D
(this is like you making $1m annually and donating $62.50)
Brin didn't go to every high school: he went to the one he did.
And maybe he had a terrible experience and thought it contributed nothing to his success... but that's kind of a dick perspective at a certain level of wealth, especially if a school has needs (and they always do).
In anything less than a fully-equalizing society, philanthropy still has a place.
(Said as someone who thinks higher wealth brackets, including my own, should be taxed more heavily)
The parent commenter put it well, philanthropy is just the rich convincing [America] that things are fine.