Escaping slashes in can leave regex expressions difficult to read. A regex statement that is meant to identify a url like this one http://atdc.org/wp-admin/admin-ajax.php.

would look something like this using the standard /expression/ notation:

/http:\/\/atdc.org\/wp-admin\/admin-ajax.php/

But if you use the %r{expression} notation, then your regex expressions do not need to be muddled with escaping slashes:

%r{http://atdc.org/wp-admin/admin-ajax.php}

Much easier to read :)