It is personal taste rather than the product of some argument about correctness but I really like shirt jackets for knocking around. There's a little less weight on my shoulders and I stand out a bit less at the grocery store, but I still have pockets and a covered waist. The pockets are a big advantage over a cardigan sweater.
Shirt jackets are comprised principally of straight seams and are usually made cost-effectively at a shirtmaker rather than a tiaior. The version in the photo was sewn by MyTailor from unlined 14 ounce/420 gram Scabal linen. It is worn with a DJA royal oxford cloth shirt, a Cravate Royale ascot, twill trousers and G&G monkstraps that have been darkened with black polish over time.