Socket testers can be quite useful when PAT testing; although some PAT testers will check the supply socket for polarity etc, not all do and it can be useful to check the socket before testing. I find them particularly useful to check the remaining sockets on an extension lead after the PAT test. You simply unplug the extension lead after the PAT test and plug it into a mains supply and plug the tester into the remaining sockets on the extension lead thus ensuring that all sockets have been tested.