Being able to specify a language outside an implementation is extremely useful to prevent hidden logical inconsistencies between different parts of the language, and makes the language more robust.

It also allows people to design new backends (looking at CUDA LLVM backends)by finding out the right abstraction to support performance. For example, implementing a C or C++ compatible CUDA backend required the C++ committee to make changes to the memory model / consistency guarantees of C++ atomics. If C or C++ had only depended on compiler implementation for it, then there would have just been different implementations with different guarantees with no consistencies between them, and no single way to even define why they were different.