Perhaps you're comparing a multithreaded version with a single threaded version, or a single-host build vs distributed build.
When adding new features to a compiler, you might want to verify that the old and new versions have the same output given the same input. If your compilers produced non-deterministic output, this exercise would not be possible.
I took the statement "different compilers" to be completely different projects and codebases. Of course reproducable builds make sense in that situation. But why in the world would you ever expect gcc to 100% always match the output of llvm? That doesn't make sense.
When adding new features to a compiler, you might want to verify that the old and new versions have the same output given the same input. If your compilers produced non-deterministic output, this exercise would not be possible.