In our company Epics stand for features. Those features extend over more than just one release (version). As there are huge features we would like do divide those into smaller sub-features to which we associate our Stories. Those Stories can still have even smaller Subtasks (which represent actual defined code).
For now it is not possible to associate Epics to another Epic (via Epic Link) for having kind of Sub-Epics,but we would love to be able to.