"I think it's good for him to have something to do." He doesn't know how much Robb has told her, so he won't go into detail, but Robb seems to need to have something to keep his mind off of what happened before he arrived here. Something that doesn't also involve drinking, which he knows Sansa has brought up with Robb. (The tavern might not help with that part, come to think of it.) "He knows how to be in charge of something, too, which also helps."