I have been using PS for years, and when you in PS perform the Save for Web action with "cmd+alt+shift+s" you see a preview of the image that will be saved. When you in Affinity Photo use the same command for Export you DO NOT see any preview of the image that will be exported. So why its this so important to have?!:
Im saving images to used online 99,9% of the time. That means i want to have the best quality possible, with the smallest file size possible. In PS and JPEG export you can lower the quality, to reduce the file size, until the image quality is no longer acceptable. Then save. In Affinity Photo you can do the same, but with the big difference that you can not see how the image will turn out until you have completed the export!