It allows you to read from stdin and grab the data that usually would be sent by mail.
To get this to work on Windows I needed to create a batch file that calls PHP with the PHP file as parameter:
c:\apache\php\php.exe -q "c:\www\maillogger.php"
And finally the path to the batch file needs to be set as sendmail_path in php.ini.
sendmail_path = "d:\www\maillogger\maillogger.bat"