a button which can only do one thing
That's a self-imposed, unnecessary limit, or incorrect assumption. A button, even with only one action listener assigned to it, could do multiple things.
Perhaps one action listener is common to multiple components, providing the ability to do the same thing multiple ways, resetting the program to a previous state, for example. While multiple buttons may perform this action when pressed, they may each have their own specific function, closing a dialog, or reading an input field, etc.
Knowing that one component can be assigned multiple action listeners is the important point. You may someday find that useful.