Not only that, but we currently suffer the 'all eggs in one basket' pathology. Namely, if something catastrophic happens to Earth, we could find the very survival of the human race at stake -- something Steven Hawking has been alerting us to for some time now.
While no existential threats appear to loom on our horizon, we know that the Earth has undergone numerous mass extinctions in its rich history, so it would be the epitome of hubris to think that humanity was invulnerable to nature's undistinguishing wrath. A vast collection of asteroid colonies would mitigate any threats to the human species posed by an extinction event on Earth.
DNEWS VIDEO: Hawking: We Need to Leave Earth
Once the asteroid belt has been explored and settled, the rich resources of the Jovian planets could be harnessed, further broadening our capabilities and influence throughout the solar system.
With gigantic ice fields, Europa might be a target since any inhabitants could, conceivably, burrow beneath the ice for protection from the Jovian radiation. In addition, the ice would provide a virtually unlimited reservoir of drinking water for the inhabitants, not to mention oxygen (for breathing) and hydrogen (for propulsion).