We don't know if it'll give exp or not yet, but overall I wouldn't be surprised if they didn't. At least not without some nerfs because it would be a touch too far on the exploitable side to just run the patches over and over again when most don't take long to complete at all.

As for why they made it if exp isn't available, assuming it isn't, it's to give people who skipped the story or whom want to redo the story the ability to do so. Lotro has a very similar feature and in games where the story IS one of it's most important features it's something I'm amazed wasn't implemented sooner.