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.