In this case, "could use" means "need" or "require". B.) is saying that the potatoes need/require (would be better if they had) a little salt. If you want to say the potatoes are a little salty, you would use "have" or "are". Just a note: B.) should really reply with a "yes" or "no" to the question about the lamb before complaining about the potatoes. A : how do you like the lamb? B : It's good, but the potatoes could use a little salt. or B : It's good. The potatoes could us a little salt though.