"It's the hope that's important. Big part of belief, hope. Give people jam today and they'll just sit and eat it. Jam tomorrow, now -- that'll keep them going forever."
Like so much Pratchett, this makes me smile, because it's funny and it's funny because it tastes true. The idea of jam (or whatever you crave) in the future has a particular draw that having the jam in hand (or mouth) doesn't. It would seem like having it today would be clearly better, since tomorrow never really arrives, but no.
Somewhere in understanding this twisted truth that going-to-have-it-later is more shiny than having-it-now, is, I suspect, the key to something Very Important.
So what is there about jam tomorrow that makes it a more powerful incentive than jam today?