The fact that the interface is implemented IS the tag. It is just liek some sort of boolean property on the class that doesn't really take up data in the class. At least that's how I understand it. E.g. For Serializable, if the serialization APIs can cast the Object to Serializable the they know it is ok to serialize the class. Because the APIs get a generic Object there is no data member they can check or method they can call unless EVERY class implemented that member/method. So interfaces are a usefull may of tagging.
It allows you to test a class easily for that property. Using SingleThreadModel from servlets as an example, if a Servlet implements this Interface then only a single thread can access a Servlet at a time. Now you can write some code like this:
So this is a pretty simple example, but it means that the class you wrote has an extra (boolean) property associated with it that can be tested without any properties or additional code or difficulty. Quick and easy flag to say 'this class is different' Dave.