Installation should ensure that all necessary files are available for installation and use.
Example: If ajaxCalls.php gets deleted, and the server is configured to redirect 404s to the index, and the index automatically redirects to install.php, you end up in a request loop that won't die and it's not obvious why the communication failed.
If the installer automatically checks for the existence of various files via PHP's file system, you skip introduction of any issues due to server rerouting.
Installation should ensure that all necessary files are available for installation and use.
Example: If ajaxCalls.php gets deleted, and the server is configured to redirect 404s to the index, and the index automatically redirects to install.php, you end up in a request loop that won't die and it's not obvious why the communication failed.
If the installer automatically checks for the existence of various files via PHP's file system, you skip introduction of any issues due to server rerouting.