Celebrity charities and fundraisers are controversial because they are seen as publicity events to boost a celebrity's profile, rather than an altruistic event. That being said, no matter the intention of the celebrity, the event still generates money, which does make a difference for those who are on the receiving end of the money.
The fundraisers and charities held by celebrities make a difference because they generally have large sums of money available to donate. The existing publicity the celebrity has helps get the word out about their organization, and supplies a face to a cause. People are easily influenced by the media, and when there's a famous person backing a cause it's more likely that the public will donate to the charity.
Celebrity charities and fundraisers really make a difference, but not because of the celebrity's donations. The difference that the fundraisers make is primarily that of raising public awareness of the cause. The publicity elicited by the celbrities' presense increases press coverage of the charity and educates the public about the existance of the charity and the cause it promotes, leading to more donations from the general public.
Celebrity charities make a little bit of difference but the fundraisers do not. Most of the money raised goes towards throwing the fundraiser and paying the people who run and work it. Very little actually gets donated to the charity. The fundraisers are just an excuse for wealthy people to get dressed up and go to a fancy dinner and dance and show off how much money they have.