Serial killers like taking tokens of their victims. I suspect that Avila had kept a piece of clothing or another item--like jewelry--of Samanthas. If Mom could ID it, it'd tie him to Samantha.
I'm listening to a rerun of Sheriff Carona talking about Samantha and talking to Avila. He's one of the finest law enforcement officers I've seen. He reminds me of the sheriff in the Susan Smith case.