For an alternate view, see Robert Zubrin's powerful case for Mars exploration and colonization . The big advantage is that all the material resources necessary for life may be found on Mars. Early settlements can be expected to orbit the Earth, or perhaps our Moon, for easy communication with and transportation to Earth. Later settlements can spread out across the solar system, taking advantage of the water in Jupiter's moons or exploiting the easily available materials of the asteroid belt. Eventually the solar system will become too crowded, and some settlements will head for nearby stars.
Interstellar travel is impractical due to long travel times.
If energy and make up materials for the trip can be stored, a stable population can migrate to nearby stars. At the new star, local materials and energy can be used to build new settlements and resume population growth. With great difficulty. Building cities in space will require materials, energy, transportation, communications, life support, and radiation protection. Launching materials from Earth is very expensive, so bulk materials should come from the Moon or near-Earth asteroids where gravitational forces are much less, there is no atmostphere, and there is no biosphere to damage.