Picture a rescue scenario where the victim is perched precariously above a high ledge or falls.  Do you really want to take a chance going over the falls if you slip - probably not.  This is what v-lowers are absolutely great for.  Your teammates place you where you need to be with pinpoint accuracy and safely.  Since you are very secure, you can work with both hands to free the victim and securely grab them.  Lots of hands on shore can safely retrieve the two of you.  Two independent lines also provides an extra margin of safety.