I have always assumed (with no concrete evidence to back my assumptions up) that most celebrity charities are just ways for the celebrity to put some his or her friends or relatives on the payroll without having to spend his own money.
ETA: Link to column: What’s up with celebrity charities? - The Straight Dope – Rico
Usually, methinks, the celebrity her/himself provides a huge contribution to the charity, so it’s not exactly “without having to spend his own money.” He spends his money by contributing it to the charity, which is (up to limits) tax-deductible in the US so long as the charity has appropriate tax-status. Note that, if he put relatives or friends on his payroll, he could still take the payments as a tax-deduction (assuming he had incorporated himself.) So, there’s no real tax advantage to setting up a charitable organization to provide employment to your relatives. The tax advantage is that the charitable organization is tax-exempt, whereas if you incorporate yourself, you are not.