The Tester plugin is only useful when trying out other plugins; as a user or as a plugin-developer. What Tester does is it lets you connect to different inputs and force the Tester plugin to render different states on screen, for example an error state. This is good to have if you are trying to figure out if a plugin of yours are really transmitting something: just drag a connection from the output of your plugin and attach it to, for example, the error status input. Every time the Tester plugin receives a signal on that input, it will turn red for a predefined amount of time.
Read more about the Tester plugin here