I can't think of a single reason to send people into space. No manned mission will ever be self-sustaining, and therefore will always be a drain on taxpayers. In contrast a number of types of unmanned spacecraft actually earn revenues or otherwise pay for themselves through cost reduction for businesses, for example:

- telecommunications (enormous number of customers)
- weather satellites (saves lives and prevents damage to equipment such as sea-going vessels)
- earth observation satellites (monitoring of agricultural productivity, pollution, forest fires)
- global positioning satellites (enormous cost savings due to accurate navigation & shipping)

Other NASA programs & spacecraft that have proven to yield greater results for the cost compared to manned missions:

- ground-based telescopes that search for near-earth asteroids & comets
- space telescopes that bring us pictures of astronomical phenomena
- research on exotic aircraft geometries, propulsion systems
- probably tonnes of other stuff

Personally, I'd rather work for the Department of Energy. But I'm Canadian, and I'm not even sure we have one (too much American tv programming - but not really actually :-)