The most popular theory is that the Eternal is in some way meant to ensure the survival of the Pure Dalek race. However, I think that is exactly why Moffat will have a different purpose for it in mind.
One thing is almost absolutely certain, we will see the Daleks again in season 7. I think Moffat was keen to make season 6 a dalek-free season, but next season he will want to do something with this new Dalek paradigm. And it is my hope that he will have some new ideas to bring to the 40+ year old story of the Daleks in Doctor Who.
When the Daleks keep getting whupped every season by the Doctor, it erodes their aura as an arch-enemy. They are meant to be exceptionally cunning, morally devoid, and almost invulnerable. They are a time-travelling species, who took on the ancient Time Lords, in a war that most lifeforms were unable to perceive. Sure, there is always a funny side to the Daleks, their absurd voices and contrived plans, but they deserve better than to be trundled out as a generic end-of-season threat.
There's still so much about them that has never been explored - when and how did they become a time-travelling species? Will we see the 11th Doctor encounter one of the earlier incarnations of the Daleks? If he's travelling in time, and they're travelling in time, it doesn't make sense that he'd always encounter the latest model.. and will we ever see them perpetrating some brutal invasion or ruthless scheme that has nothing to do with Earth or humans?
Anyway, Eternal Dalek... I think this Dalek's role is to distract all non-Daleks as a prelude to invasion, by engaging them in a never-ending internet discussion about its nature, hence the name Eternal Dalek.