For highly reliable code, knowing that the code you review is the code you compile with is vital both for stability and security. This can't be done by visual inspection: it requires good provenance at every stage of the game.
This is actually a security problems with many opensource and freeware code repositories. The authors fail to provide GPG signatures for their tarballs, or to GPG sign tags for their code. So anyone who can steal access can alter the code at whim. And anyone who can forge an SSL certificate can replace the HTTPS based websites and cause innocent users to download corrupted, surreptitiously patched code or tarballs.
I'm actually concerned for the day that someone sets up a proxy in front of github.com for a localized man-in-the-middle attack to manipulate various targeted projects.