Edit Poly modifier is just a way to pass some of the modeling functions you get in editable polygons up the stack.  There are some operations that can't be passed up as a modifier, so you will find there are things you can't do in the edit poly modifier that can be done at the base level. However, there's some things very useful about the edit poly as a modifier as well, like the ability to maintain animated information, or to "hold" settings (in animated mode), which allows you to go back down and adjust those parameters later.

For example, if you set the edit poly modifier to "animated" mode, instead of the default "model" mode, and add any kind of modeling operation, like say an inset, you can add other modifiers to the stack, do more changes to the object, but the parameter settings of your inset on the edit poly modifier are stored eternally.  So, you can go back down to that modifier, click on the inset again button again, and actually re-adust the settings at any point in the creation process.  As long as your stack is intact.

This can be very powerful for visualization, for example, or for any kind of presentation purposes; you can show clients what the results would look like, for example, at different parameter settings, which could result in a different object appearance.  If you have multiple edit poly modifiers in your object, set to "animate" mode, you have the power to adjust whatever operation you did on those modifiers, even if you have them keyframed.  It's been refered to as the "ultimate undo", and while there are some benefits also to a modeling history, as in other apps like Maya and Cinema 4D, the procedural nature of the modifier stack offers certain functionality not possible any other way. 

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