The asteroid mission would act as a "stepping stone" for future planetary missions to Mars and beyond. This three-month trek would provide vital technological, psychological and practical clues to what a manned deep space mission would face. Landing on an asteroid will be very difficult (due to the tiny influence of gravity on such a low-mass body), but it would provide an opportunity for astronauts to mine for water ice, use it for consumption and convert it into its component hydrogen and oxygen (for fuel and breathing). These tests would be essential before sending man on a long-term mission to Mars.
Wait -- I thought the Moon was the logical "stepping stone."