Shouldn't NASA's role be more blue-sky research for things we don't know yet are feasible or possible?
Another motivation is a desire to understand the design principles and behavior of such concepts at a deep and general level. That's not usually something industry does—often industry's level of analysis and experimentation is just "good enough" to get a job done. If that gets repeated a few times, it leads to both wasted effort and a proliferation of mediocre designs. (As another example, that motivation also drove the National Advisory Committee for Aeronautics—NASA's predecessor agency—to undertake an exhaustive study of airfoils. NASA still undertakes quite a bit of research from a similar perspective.)
Discussion here: https://news.ycombinator.com/item?id=11329286
At some point, all the science will get done and the work will slide over to more engineering type work and we'll end up with a neat plane/gun/database/internet/drug/medical test/etc.