B) (Related to A but worth mentioning separately:) Because the space program has led to major advances in medicine that have improved length and quality of life on Earth. [1]
C) To gain access to essentially limitless energy resources via space-based solar.
D) To gain access to material resources (e.g. minerals) that are scare or absent on Earth. [2]
E) To create an off-Earth breeding population of humans so that one asteroid / plague / nuclear war / whatever cannot wipe out the entire species.
[1] Here's a simple example: When the Hubble telescope was initially launched, it had a small lens defect that blurred the pictures slightly (the pictures were still better than any Earth-based 'scope, though). Image processing software was developed to clean up the pictures until a replacement lens could be deployed. That software then migrated over to MRI machines where it is now used to detect tumors much earlier than used to be possible.
[2] These resources are better accessed from the asteroids than Mars, but going to the latter gives you access to the former.