On Zero Jacket Review: Testing the 61g shell that fits in your pocket 

It’s a good point. Had forgotten about race mornings at the start line.

Wind and just a bit of cold. He explained it in the beginning of the video

From ON costumer service: "To respond to your inquiry. You may use the windproof, water-resistant Zero Jacket for running, cycling, and hiking because it is incredibly lightweight. The best feature of this fabric is that it has an astonishingly low weight of only 29 grams per square meter, which reduces the overall weight of the jacket to a barely credible 52 grams. However, we do not advise wearing the Zero jacket under a backpack as there is a genuine possibility that the fabric will tear from repeated contact, especially when jogging. Additionally, we don't offer free local repairs or free shipment to have the product repaired in any area. However, you can submit a claim under our warranty if you think the damage was caused by a manufacturing defect in the products, which would enable our warranty team to assist you further while they investigate your claim. I value your consideration and understanding."

Guess it's really for adding warmth when it's not raining. Some wind protection. etc.