What are the main reasons for using Epics?

[barbarat]

We have a few stories that have grown as we are creating sub-tasks to them (in the middle of a sprint).  We realize now the story may be better categorized as an Epic (large user story).

Is it worth changing existing stories into Epics during a sprint? In that case we would need to promote the sub-tasks into stories. What are the main benefits of using Epics?

[AgentSmith]

We have that discussion taking place here, which I am hoping will benefit:

https://answers.atlassian.com/questions/85897

Also briefly discussed here:

https://answers.atlassian.com/questions/291555 

http://blogs.atlassian.com/2012/09/agile-qa-greenhopper-time-estimates-with-sub-tasks/