You step into the shower and realize that you are out of shampoo. The only thing you have in the shower is a bottle of conditioner that is half full and a bar of soap. You don’t want to use the soap on your hair, but you could use the conditioner. It isn’t a replacement for shampoo, but it might do one time. The shampoo is used to get your hair clean from grit and grime that is in your hair and on your scalp.
Conditioner serves another purpose. It is used to make your hair smoother and more manageable. If you squeeze some shampoo in your hand and rub your hands together, it would get sudsy. If you did the same with conditioner, it would not get sudsy.