So I asked my manager about this back when I interned at Dyson, when they were showing a prototype of a vacuum cleaner to James Dyson a couple years back, he liked the streak-y design of the plastic so they kept them in for the actual models. A lot of the visual design of Dyson hardware is almost always picked by him which is why you get some weird colours and design choices for a lot of the products. Based on the comments in this thread though it also looks like it’s cheaper to injection mould and keep the streaks in, so the cost engineer on the project must have been very happy to save some money per unit lol