[zozbot234]

Knowledge is built by iterating on previous knowledge - having freely available knowledge around on how to build turbines or reactors makes it far more feasible to develop better versions of those existing designs. It's not just about the money involved in construction, albeit that could be a factor too.

[jka]

Arguably safety can benefit, too. Open designs allow anyone to inspect both an initial design, and also the modifications that are made to that design over time.