If I dare ask, why does the human race want to leave the earth? There's no reason or benefit. There are more interesting, more pressing issues having mostly to do with our biology.
There are at least a couple reasons: 1) A lot of the conflict that destroys so many lives is motivated by limited resources such as land. Getting off this planet could make that kind of conflict disappear (eventually). 2) Diversification. If climate change/oil scarcity/nano/GE get out of hand, it'd be good to have someplace else to go. Even if you're sceptical about the environmental risks from climate change and oil scarcity, GE and nano pose big risks. The easier nano and GE get, the more likely some bio-/nano-hacker somewhere creates an irreversible problem for us (whether accidentally or intentionally).
The way i see it we 're still in 'space exploration' not colonization. Perhaps if we master biology, life and intelligence first, we 'll have our drones do that for us.