Summary: They were seeing a disconnect between Arkham Asylum player stats and sales, indicating a large portion of the playerbase blasting through the game then selling it back to retailers. WB studios were directed to explore ways to lengthen player engagement, preferably enough to keep the game forever.
The nemesis system gives some light procedural flair to an otherwise-deaigned experience. I don't think it did what they hoped it would, but it was still a great mechanic.