"...
Boeing’s alternative would combine the Orion rendezvous with a pitstop for gas, allowing the Ares V to lift off from Earth with a much larger payload—and an empty lander. Boeing says this would allow NASA to deliver about three times as much mass to the lunar surface, and over fifteen times as much payload. What’s more, Ares V could then send the lander-Orion package all the way to lunar orbit with full tanks, rather than NASA’s current plan to use extra propellant in slowing down before soft landing."
http://www.popularmechanics.com/science/air_space/4224660.html?series=35

Linked from http://www.transterrestrial.com/ and written by Rand

Of course this also applies to Manned Mars missions [and robotic and satellites to GEO].
But more important than allowing much larger payloads is it could/should develop a rocket fuel market in space.