Back when I didn't give keynotes, only attended them, I thought they had two purposes. The first was to let me get near some godlike figure - your Herb Simon, your Guy Steele - and have wisdom flow from their head into mine. The second was to be inspiring.

I now realize that neither of those is to the point, though the second is closely tied up with the real purpose of the keynote, which is to provide a theme for the conference that threads through all the conversations that make it up. People should refer to the keynote in hallway talk. Other presenters should refer to the keynote's theme as they speak their piece. Questioners should ask presenters questions informed by the keynote. Collectively, the theme gets amplified and people resolve to do things about it.