@ImTheMostManlyMan: If you just play Arena (which I haven't but read enough wiki articles) you'd think it was just for power but when you read The Real Barenziah it dosen't seem that way.
Well, the Tharn family has a long history of being evil and working alongside the Daedra. In the Elder Scrolls Online, everything revolves around a member of the Tharn family who made a deal with Molag Bal. In Arena, a descendant of this Tharn traps the Emperor in Oblivion and takes on his guise. Jagar made a deal with one of the Daedric Lords that would greatly benefit Jagar and the Daedra. First off, Jagar becomes the Emperor, and has all of the wealth and power that an Emperor has. Second, the Daedra as a species benefit from this because the Amulet of Kings and the Dragonfires have no power when the Amulet is in Oblivion. This gives the Daedra and Daedric Cultists free reign to travel between Oblivion and Mundus with little to no consequences. Jagar, in my opinion, made this deal so that he could gain immeasurable power as and Emperor and so that the Daedra would reward him greatly. If I was in Jagar's position, then I would have done the same thing.
But this is my reasoning and opinion.