* It's a jobs program to avoid the knowledge loss created by the end of the cold war. The US government poured a lot of money into recreating the institutional knowledge needed to build weapons (e.g. materials like FOGBANK) and it's preferred to maintain that knowledge by having people work on nuclear programs that aren't quite so objectionable as weapon design.
* It helps you better understand the existing weapons stockpiles and how they're aging.
* It's an obvious demonstration of your capabilities and funding for deterrence purposes.
* It's political posturing to have a big supercomputer and the DoE is one of the few agencies with both the means and the motivation to do so publicly. This has supposedly been a major motivator for the Chinese supercomputers.
There's all sorts of minor ancillary benefits that come out of these efforts too.
* It's a jobs program to avoid the knowledge loss created by the end of the cold war. The US government poured a lot of money into recreating the institutional knowledge needed to build weapons (e.g. materials like FOGBANK) and it's preferred to maintain that knowledge by having people work on nuclear programs that aren't quite so objectionable as weapon design.
* It helps you better understand the existing weapons stockpiles and how they're aging.
* It's an obvious demonstration of your capabilities and funding for deterrence purposes.
* It's political posturing to have a big supercomputer and the DoE is one of the few agencies with both the means and the motivation to do so publicly. This has supposedly been a major motivator for the Chinese supercomputers.
There's all sorts of minor ancillary benefits that come out of these efforts too.