The Neumann thruster is all about saving launch capacity. Most of the ion thrusters we have now work with e.g. Xenon gas; you have to loft their fuel, and your engine mass budget has to include the material handling for the propellant; tanks, valves, etc.
The idea of the neumann thruster is that your reaction mass comes from a simple sold puck which is gradually ionized; so you immediately win on a bunch of hardware you don't have to lift.
And then, you can use as reaction mass the sorts of stuff which is already up there in orbit. Got some excess second stage, which you've lofted to orbit at ruinous cost? Instead of dropping it back into the atmosphere to burn up, melt it into a puck at the focus of some mirrors, and then use it as reaction mass for a few years.
Space junk turned into valuable fuel. Big win.