I have a down vest and synthetic vest and use them both. A vest warms your body core. The theory is that if your core is warm, your arms will warm up too. The reason- weight savings. With the new very light down sweaters out there now, the rationale for a vest is a bit less compelling. I really appreciate the vest on cold mornings, but it really is not essential. My Marmot vest has elastic in the sleeves for a tight seal my upper arms. This adds to the warmth a lot. I got into vests when technical climbing, just for the warmth without restricting arm movemet, but that usually is not a problem if only backpacking.