Why would one go about creating a site such as this?

Purely selfish reasons! I know that I, at least, frequently consult the perl documentation to quickly resolve any questions about functions - parameter order, return value, etc.

Beyond that, it is quite handy to be able to spout off a url in response to any question that someone may have about a particular function. As it is derived from the original works, it can be considered quite correct.

But why would you make it in PHP? Heretic!!

I happen to feel that PHP was better suited for this job than perl was. It would have taken me longer to concoct this site via mod_perl or cgi than it did with PHP. Perl wasn't left out, however - It was a pure perl solution that transformed the original html documents on perldoc.org into the format that I needed them for this site. Perl also checked the referential integrity of the links between functions.