The idea is to allow the tool to be instrumented by other tools, for example if you have your own particular build setup. That script used to just be used for WebKitSystemInterface, which is just a Mac port thing; but now it's also used for LLVM and it will apply to any port that wants LLVM support.
Created attachment 222840 [details] the patch
Landed in http://trac.webkit.org/changeset/163193