It's there to stop more than one instance of the plugin loading into the menu/context-menu etc.
If you hadn't added this trap then this might happen if you typed load "MyPlugin.rb" after you have already got it loaded - e.g. automatically at startup.
It's not going to have to stop these potential duplicate menu items unless you try to 'load' the script more than once - e.g. while you are testing it, or perhaps when you have a script in a sub-folder that doesn't auto-load, but it might be loaded by another script [that is itself already loaded] under certain circumstances, and potentially these circumstances might occur more than once in that session - but you only want the one menu/context/toolbar item...