1.) Life may be very rare in the universe. If there was a man-made or natural disaster like an Astroid it could kill all of life on Earth. If Mars was colonised - it would ensure the continuation of life (human and the animal kingdom). This could allow further intelligent life to evolve. (or perhaps more intelligent).
2.) Natural Resources are finite. Jupiter has a moon that has more natural assets than the whole of the earth.
3) Travel to other stars systems could help deal with population growth, and fuel further understanding of science and technology.