[sqeaky]

It creates plausible dependability.

Imagine the NSA publishing a crypto algorithm and contributes it to openSSL or some hypothetical crypto library using git. If they commit their new algorithm, everyone will be looking at that. They could do something devious like tinker with the way random numbers are generated elsewhere and reduce the possible keyspace of another algorithm to something very small and easy to brute force.

When this keyspace shortening is found out it would be hard or impossible to track back. No amount of inspecting the files that reportedly changed would reveal that the NSA did this.