Of course given the libraries and systems already available, you don't practically need a lot of this knowledge, but it does help. When certain portions of an application just don't perform well, or when you hit really weird side effects of race conditions on static properties etc. I think it's also important to know the language and tools that you are working with as well as possible. It's not always possible to know everything, but you should probably read and complete at least one book on the language you are using. You can hack away in any given language for years. But until you've actually read cover to cover on a comprehensive book, you won't necessarily understand some concepts, or better still you will learn how the language does something for you, that you've been doing the hard way.