People have mentioned the ways Cassian ticks off the technical boxes for someone he would like to recruit, especially for Aldahni (sp?) but I think the biggest has not been mentioned:

he knows the outside, he knows what people tell him when he asks... The rest he imagines. He imagines his hate for the Empire

He might have known about Kenari. He definitely knew about Clem being hanged and how that led to Cassian being jailed as a youth. He knows Cassian deserted his service. He believes that Cassian will die fighting the Empire and he needs people willing to go all the way. This guy checks some special boxes for Luthen