Why look, here's one of the dogs they placed for adoption before the fire!
Anyway, as far as I know, the people who got my tickets were simply eager to see Robin Williams, no matter who the event benefited. With that kind of star power, the event itself is practically a guaranteed success.
But the more interesting question, given that not every organization will be lucky enough to sign up Robin Williams, is whether those guests who were just there for the fun went away with some interest in supporting the Humane Society in the future? I'll never know for sure, but it's an interesting lens through which to view your own events. How do you make sure your message will reach not only your loyal supporters, but friends of friends, substitutes for people who stayed home sick, and so forth -- who may have only a dim idea of what your organization does?
Putting up posters and other visual representations of your organization's work is a good start, as is a rousing speech during intermission. I believe the Humane Society brought a dog along. In any case, remember that such an event offers a rare chance to compellingly and succinctly state what your organization does, why it does it, and why someone who just walked in off the street should care. Let your message outlive the event!
Now, back to my cup of ginger tea.