If you upload high resolution photos, and someone is scrolling through their feed on mobile data, downloading those photos is going to consume a large portion of their bandwidth. Proxying them through allows the hive front-ends to reduce the size of an image when it's previewed, saving you huge amounts of bandwidth.
images.hive.blog / https://images.hive.blog
The proxy also supports changing the image format. This is necessary because not every device/browser supports all image formats. That way you don't have to care about browser compatibility when you upload a picture, because the proxy will do so instead.
Yeah, that's the point I have missed. We do need it, thanks for explaining.