The Spin-off Argument
From a medical standpoint, living in space is harsh on a body that evolved to walk upright and relies on a certain gravitational pull. Travel to other worlds for an extended period of time will require technology that can repair bone damage, strengthen and reinforce muscles, properly distribute necessary bodily fluids, replace irreparably damaged organs, and defend the astronauts from radiation.
Not only that, but it must be capable of doing it all as non-invasively as possible since surgery in space would be too much of a risk for all but the most critical cases. All this is technology that can be applied to real world medicine and used at a hospital near you to help more patients recover from debilitating injuries, disease, or organ failure. The makers of such medical devices stand to make tens of billions annually and save tens of thousands of lives.
We can take a look at infrastructure through the same prism.
Living on another world means having to come with your own power generator, your own habitat, and your own food, water, and air supply. If astronauts could use highly portable, modular, and scalable power grids and self-sustaining habitats, why couldn't we apply the same technology to build smarter, more efficient cities with smaller footprints and fewer energy demands?