To learn about their degradation modes and how to maintain them (since full-scale nuclear tests are now verboten, and subcritical experiments only deal with fission part of the entire assembly -- we now also need some experimental setup to test the fusion part: radiation pressure, X-ray reflection, ablation modes etc. NIF is this setup).

Also, there is a constant need to improve fusion/fission rate in the total energy output, and perhaps eventually design pure fusion weapons, though this is still probably out of reach.