Tom's right about the value of a frontier to a healthy society. I'd define a frontier as the transition zone between exploration and settlement, and, noting that exploration comes before settlement, I'd suggest that settlement makes a powerful ultimate goal that advances many other prerequisite goals, like exploration and scientific discovery.

Settlement has powerful emotional appeal on many levels. In America it evokes our particularly vivid cultural mythology about the settlement of the Western frontier. On a more prosaic level, the prospect of ownership of something tangible in the future helps people make an emotional investment now in the exploration required to get there. They get at a gut level the concept of investing in space exploration because it has a definable reward of something deeply important to the human spirit: A new home. Not as "sexy" to us as the exploration of space and the thrill of scientific discovery (and warping through space and zapping Borg), but that humble emotional drive will motivate many more people to support the necessary exploration.

I think that past attempts to define the rewards as monetary (spinoffs, resource mining, He3...) really belly-flopped because the motivation of profit is too strong and sharp, compared to deep and primal like the instinct to make a home on the frontier. It challenges people too much to come up with alternatives here on Earth that will provide a more immediate payoff, or don't require the initial upfront investment. Like solar energy from space--people got too creative and were inspired to think up cheaper alternatives like solar turbine farms and better photocells.

Go with the nesting urge. Or with humans, I'd call it the "walk-and-nest" urge. That's what we've always done, picking up stakes one day and walking off over the horizon to settle down somewhere else for a while, only to pack up and move--or our descendents pack up and move--once more. Go with the flow in the gene pool.