Extensions are how new features come to an established language... C11 standard mostly standardised stuff that was already supported as extension by most compilers. Most of the time, extensions are created because features are missing from the language (alignement requirement for example), compiler could do a better job at optimising the code with better hints (noreturn, restrict, strict-aliasing, ...), or we could simply make the job of the developper a bit simpler/safer (nested functions, _Generic, blocks, ...).

IMHO, the main benefit lost by using bleeding-edge extensions is portability, which may or may not be an issue.