Networking is one of the likliest reasons. A Clinton dinner at 10k per head will attract a certain kind of audience. The speech itself may not carry as much importance as the presence of other dignitaries, industrialists, upcomers, lobbyists and so forth.
+ People get to say: 'I saw so and so'
+ It establishes the cred of the audience, i.e. all the bankers can talk to clients about how they say/met so and so.
+ It's an indirect way to contribute to campaigns without it technically being a campaign donation
+ It's a form of buying favour
+ And yes - it attracts other people whom you might be interested in meeting with etc..
