Many reasons actually. You more than likely have something on you and use something daily that only exists due to things implemented for space travel. Those "side products" really add up, since space is so frigging hard on everything, which forces human innovation to go beyond the current limits. That new ground is what fuels innovation, fuels economies, fuels wealth. And that in turn should power things like space travel, since that helps growing the space for innovation even further.

While I could point out obvious stuff like PV-panels or smartphones, how about thermoacoustic refrigeration? Invented for the James-Webb Telescope, they are currently evaluating entire new fields where this cooling method saves lots of power - remarkably, the Telescope was planned with this technology even though it did only exist as an idea at the time, not even as a working prototype. That created lots of jobs and now keeps creating new ones as it opens the field for cooling things to near-absolute zero that are highly sensitive to vibrations. They are still trying to understand the technology, since it works.