In Software, complexity naturally increases over time and dependencies and interactions between components become impossible to reason about. Eventually this complexity causes the Software to collapse under its own weight.
Truly reproducible builds (such as NixOS and Nixpkgs) provides us with islands of "determinism" which can be taken as true invariants. This enables us to build more Systems and Software on top of deterministic foundations that can be reproduced by others.
This reproducibility also enables powerful things like decentralized / distributed trust. Different third-parties can build the same software and compare the results. If they differ, it could indicate one of the sources has been compromised. See Trustix https://github.com/tweag/trustix