The short answer is, Yes!
The long(er) answer is, Yes, you do!
Who really thinks about what would happen if they couldn't make a medical decision for themselves? From a doctor's appointment, to simple surgery, to ongoing care, a Medical Directive (also referred to as a Medical Power of Attorney) can be your key to spelling out what you really want to happen as it relates to your medical care. This means the person you choose to act on your behalf doesn't have to guess what you want--they simply follow your instructions.
So, YES, you need a medical directive.