It's great for asking "who uses this function" and "how do they use it"?
CPAN is used by the perl community to make lists of modules broken by new perl releases. They use this to find how intrusive proposed intentional changes are, as well as to find accidental breakage.
Take it further - they can look at all the apps in their App Store and make decisions like "Okay, lets improve this instruction set in the next ARM processor we make".
