The nature of life is to spread, adapt, and fill new niches. The "don't put all our eggs in one basket" philosophy was and is one of the oldest and most pursuasive arguments for settling space colonies-- if successful, just one self-sustaining off-Earth human gene pool doubles our survival chances as a species. And the more colonies we make, the greater the odds of some humans somewhere surviving.
I don't know if that means it's "necessary" to do so, but it is a very good idea.
STARGAZING: All I see are the lights of a billion places I'll never go. --Howard Tayler, Schlock Mercenary