Generally so that you are not affecting other projects that may reside on the same server (using the same config file).
Side note and sort of self promotion, have a look at https://github.com/KyleWolfe/PHPErrorNet. A controversial topic, but like it or not, PHP is far looser than other languages. I recommend tightening it up, and force yourself to write better code. This project also gives you an easy way to have centralized reporting on those exceptions / errors (email, logging, etc)