The name comes from advertising, where the last frame of a commercial shows the pack. In ecommerce it is the main listing image and the thumbnail in search results, so it makes more first impressions than any other photo. Marketplaces set rules for it: see white-background photography for Amazon's.
A good packshot is plain on purpose. The product sits centred, lit evenly, with a soft contact shadow so it doesn't float. The hard part is consistency: every product in the range needs the same angle, crop, shadow and white, or the category page looks as if it came from several different shops.
