Manned space flight
It is sometimes said that we must keep humans in space in order to prepare for a manned mission to Mars. But such a mission would serve no worthwhile purpose either. Anything it might discover could be discovered at a fraction of the cost by unmanned probes.
It has even been argued that we must press ahead with space colonization as a sort of hedge against extinction in the event of some global catastrophe that wiped out the human race on Earth. It is hard to imagine a more unconvincing position. It would be a very long time before such a space colony was viable enough that it could permanently survive completely independently of Earth, let alone carry forward human civilization (where?) if Earth were out of the picture. The money and brainpower required to create such a colony would be far better invested in strengthening Earth's defenses against whatever disasters might threaten it.
Of course I believe that humans will spread outward from Earth eventually, when the technology exists do so at acceptable risk and to exploit other planets in a cost-effective way. But that time is not now. Today, we are sending people into space in dangerous contraptions at huge expense for no good reason. Space will still be out there fifty years from now. We need to learn how to walk before we try to run.