It's light, durable, and gives vastly improved edge control. Skiing XC skiing demands a fine balance between staying warm and overheating. Overheating is a major concern because you will sweat and iif you don't get rid of that moisture, you are going to be freezing cold as soon as you stop. Cross country skiers often dress like runners. But, everybody's comfort level is different - what's good for you may be freezing or roasting for someone else.
Skiers often carry a small pack with extra layers.
A small pack is also a great place to carry water bottles and snacks. In order to stay warm, the layer next to your skin MUST stay dry. Synthetics like polypropylene move the perspiration away from your body and into the outer layers. Don't wear cotton. Wool is OK too, but poly is much better.