I've given talks where I think at the time that we'd have been better off giving everyone in the audience a cheque for $500 to read our blog post rather than fly me to the US for a week.

But then one person in the audience likes what they hear or asks a really insightful question, and then next year they're giving their own talk about your work because they're using it and you realise it was worth it.

And I think a counterpoint to this, as a conference attendee, is that if you walk by a room and it's embarrassingly empty, then jump in, make eye contact with the speaker while they're talking, and ask a good question afterward, to help them out.