The important thing to know is that the NIF is about nuclear weapons design (verification of modeling software used for nuclear weapons design), not about developing fusion power plants.
Note that @dang doesn't actually notify dang, unless he happens to open this article and see it out of the corner of his eye or something. If you want to report a frontpage dupe, emailing the mods using the footer Contact link is an efficient method (or you can just flag it as I did, which has relatively the same effect if enough people do).
The fear is that the nuclear weapons sitting in storage are assumed to be functional, but might turn out not to be. This could have dangerous geopolitical consequences. We can study how the materials age, but have to use simulation to understand the effects on the detonation given the test ban. This research helps calibrate the simulations.
The civilian applications of the facility are secondary, as others have pointed out.
The US military probably thinks so, but I believe a major goal of NIF is the ability to model whether the ones the US already built will still work without setting one off.
The important thing to know is that the NIF is about nuclear weapons design (verification of modeling software used for nuclear weapons design), not about developing fusion power plants.