Because both LLVM and Clang are librarified from the off, so you can hook them into your own code and use as much or as little of them as you want. You can insert your own passes, make your own drivers, add your own analyses to the static analyzer (http://llvm.org/devmtg/2012-11/Zaks-Rose-Checker24Hours.pdf). Or use it to get information to show in an IDE.
