The syntax highlighter currently auto-detects the language inside the codeblock. However auto-detect isn't perfect (it's never going to be), and sometimes you paste a language that isn't supported and sometimes you don't want any highlighting at all. Like when you paste the error output from a compile.
In BB code this can easily be done with an argument to the code tag. In markdown it's trickier (Stack overflow uses a special tag: "<!-- language: lang-C -->" to set the language).