You should regard it as 99.9% likely that the electromagnetic thruster that allegedly does not rely on reaction mass, tested by NASA, does not actually work. The bar of proof here is extremely high.
But it’s worth talking a bit about why it would be so amazing if it were in fact possible to create such a thruster.
There’s a lot of cool stuff out there in space. A single metal-bearing asteroid might contain tens of billions of dollars worth of metal. Giant solar power satellites could generate energy with few or no environmental concerns. Obviously, there are lots of commercial uses for Earth satellites (communication, GPS, monitoring weather, mapping, etc.). And, of course, there’s just plenty of pure science to be done.
But anything that we do in space is immensely costly. Why is it immensely costly? Mainly because it’s really, really expensive to blast stuff up into space, outside of our gravity well. Secondarily, because once you’re in Earth orbit, it’s also expensive to go anywhere else, if you need to.
One of the reasons that it’s expensive to go places in space is a sort of nasty feedback loop of rockets. To make a rocket go further, you need to bring more fuel for the rocket (and “fuel” here is mainly actually about reaction mass. The big issue with rockets is not generating power like electricity, it’s creating high-velocity gas to fire out the back of the rocket). But fuel is massive — so it weighs down your rockets and makes them less powerful. The farther you want to go, the more mass you have to bring to start with, and thus you need even more fuel to lift that mass at the start of your trip.
This is the single biggest cost associated with space exploration. It’s hard to overestimate how much the physics of rocketry holds us back from any space ambitions.
So these thrusters, if they really work, which they almost certainly do not, offer a mode of travel in space that does not require reaction mass. It still requires electricity, but electricity is easy to get in space — solar panels work very well (at least until you get far enough from the sun. And even then, nuclear power of various kinds is a possibility).
These thrusters aren’t nearly powerful enough to actually lift off from the surface of the earth — not by a long shot. We’d still need to use rockets for that. But instead of our rockets lifting, say, 1,000 kg of satellite plus another 1,000 kg of additional rocket fuel for the satellite to use to maintain its orbit or jump up to a higher orbit, we could just lift 1,000 kg of satellite — and then, once we were in space, use these weak little thrusters to maintain orbit, climb to higher orbits, or even maneuver around in trans-earth-orbit space.
If you could do that, you basically halve the cost of that satellite — because the satellite itself is of very little cost compared to the cost to get 2,000 kg into earth orbit.
The bigger the thing you want to get into orbit, and the farther form Earth you want to go, the more cost savings you would realize from a reaction-mass-less thruster.
Now, don’t get me wrong. It’s very, very, very unlikely that these thrusters actually work. But if they did, they’d be a very big deal.