Most fusion attempts try to keep a continuous reaction ongoing.

Tokamak reactors, like JET or ITER do this through a changing magnetic field, which would allow a reaction to keep going for minutes, the goal is somewhere around 10-30min.

Stellerator reactors try to do the same through a closed loop, basically a Möbius band of plasma encircled by magnets. The stellerator topology of Wendelstein 7-X was used as VFX for the closed time loop in Endgame. This complex topology allows the reaction to continue forever. Wendelstein 7-X has managed to keep its reaction for half an hour already.

The NIF is different. It doesn't try to create a long, ongoing, controlled reaction. It tries to create a nuclear chain reaction for a tiny fraction of a millisecond. Basically a fusion bomb the size of a grain of rice.

The "promise" is that if one were to just repeat this explosion again and again and again, you'd also have something that would almost continually produce energy.

But so far, the NIF has primarily focused on getting as much data as possible about how the first millisecond of a fusion reaction proceeds. The different ways to trigger it, and how it affects the reaction.

The US hasn't done large scale nuclear testing in decades. Almost everything is now happening in simulations. But the first few milliseconds of the ignition are still impossible to accurately model in a computer. To build a more reliable and stronger bomb, one would need to test the initial part of a fusion reaction in the real world repeatedly.

And that's where the NIF comes in.

You can always run few extra experiments to make your hydrogen bombs smaller or more reliable.

Currently fielded devices haven't been tested, all you can do is to do some non-explosive tests and simulate the rest. Data from NIF are used in the latter