The purpose of NIF, and it’s not hidden, is to maintain the existing US nuclear stockpile since we can no longer rely on using underground nuclear weapon testing to ensure they still work. There’s a very big supercomputing capability funded under the same effort. Instead of testing the weapons by exploding them underground, we use computer modeling with the modeling validated (ie backed up) by experiment (at NIF) to make sure the stockpile works and can maintain its strategic deterrent. The euphemistic name for this is “stockpile stewardship.”