Presumably the self-sustaining settlement of other planets would be the eventual payoff, but first we have to find them, and as there doesn't seem to be anything else that is 'habitable' without a good bit of work in the solar system, having a look around the neighborhood seems like a reasonable next step. A telescope array with a significant baseline, undisturbed by transmissions from Earth, would give much better resolution than anything we have available at the moment, and make it a good bit easier to spot planets around other stars, and potentially gather some information as to their composition. While we're having a look-see, we can try some things with off-world settlement on the near side of the moon and see what works and what doesn't.
I do enjoy 'cool science,' but human space exploration is resource intensive enough there really needs to be something more than 'we carved our names up there' to come out of it.