Silva was brought in for the same reason that Freddy Garcia, Bartolo Colon, and Kevin Millwood were brought in, to be a veteran arm they could throw out there in the wake of losing Cliff Lee and Andy Pettitte. Silva was released by the Cubs and likely on the word of his former pitching coach, Larry Rothschild, they brought him in.
However, Silva dealt with injuries that really never let him get rolling in the minor leagues this season. He only pitched in seven minor league games over 36 innings with a 2.75 ERA. Not bad and the Yankees would have likely at have given him a shot if not for the injuries.
At this point there was no reason to keep him though. Colon and Garcia have emerged as legitimate options and with Hughes back the Yankees even had to bump Ivan Nova from the rotation despite his effectiveness.