The reason Columbus used to leave Spain was that he thought he could figure out a faster route to India and get easier access to resources. The reason to fight a war is usually control over resources. Therefore, the reason to explore space is resources. It's just tough to articulate what can be gained by extracting resources from space because of the huge costs involved.
The physicist Stephen Hawking
expresses complex views of why to go to space, but the
main push is that the Earth won't last forever and we need to seek alternatives to continue our human race. He is quoted as saying, "I believe that life on Earth is at an ever increasing risk of being
wiped out by a disaster such as sudden global warming, nuclear war, a
genetically engineered virus, or other dangers. I think the human race
has no future if it doesn't go into space."
Therefore, the resource to gain while pursuing space exploration is a viable alternative to Earth. The trouble is the Earth we occupy right now is actually in good shape with respect to providing us with necessary resources for the next couple hundred years (thank goodness), so spending money to develop space systems is a tough argument to make politically.
Cheers,
Rob