[EvanAnderson]

The more you know about how something works the better equipped you are to handle things breaking. It's a safe bet that semiconductor physics and the gate-level construction of CPUs isn't necessary to be a good programmer, but not much further up that stack are things like understanding superscalar processor architecture, how caches work, how CPU protection levels work, etc. Knowing about those things, for sufficiently performance or security-intensive applications, can make a ton of difference.

There's an analogy to networking there, too. You don't necessarily need to know how wave-division multiplexing, BGP, or DNS work to communicate over the Internet. For some categories of problems, though, a little bit of knowledge allows you to punch just a bit above your level.