Debugging. If you don't know how your code is translated into CPU ops, you won't be able to recognize the clues that malfunctions almost always provide when code misbehaves. Translators, compilers, libraries, and new code routinely have bugs. Depending on a google search to pin down the source of your troubles often fails, and of course is of no help at all diagnosing your own bugs.

It also depends on your philosophy. I want to master my machine not be its servant.