1) Building out sufficient solar power collectors to solve the Earth's energy problems forever.

2) Manufacturing medicines, alloys, and materials that can only be produced in micro-gravity.

3) Protecting humanity against social breakdown and/or extinction in case of massive asteroid strike / global pandemic / etc.

4) Because every dollar that has been spent on space development in the US has put $7 back into the economy by way of spinoffs, technical advances, etc.