Very lightweight and easy to pack into a small size.
I was looking for a lightweight and easy to pack jacket to carry in my back pockets or strapped to my waisted carried water/fluid bottles. Sometimes you are out on your athletic venture and the weather changes or you might "bonk"/"hit that wall" or need to stop for whatever reason. Being able to add another piece of clothing can be a huge help in such a situation. As for sizing, I order one size up (XL) since I might be putting this over a long sleeve base layer and Sporthill Vest that I commonly wear while skiing.