It's Brown's utility value and the fact that he came cheap. My guess is Brown will play off the bench and come on as an impact player in the backrow (he is good at lifting the tempo), or as cover for Killer and Matai in the centres if they happen to go down. He can also cover hooker if we are only carrying one of Koroisau or Parcell. For that reason, I can understand his signing; I don't think Barrett has ever intended for him to come along and replace the other backrow options.
Brown also got unfairly judged by Manly fans based off some performances at the backend of last year...the reality was the Panthers were contending for the spoon and the place was a basket case. Lewis had a bumper year in 2014 when the Penrith made the top 4 and he was excellent for the Warriors when they made the GF in 2011. He is a very good player with plenty to offer.