While the mobile site seems sleeker the app gives more options. As Andrew was saying sometimes your default app you use to upload pictures doesn't always have ALL your pictures. Gallery, Google Drive, Google pictures, Picasso etc can host many pictures but not all of them in the same place. Your gallery may not have a picture you want to upload because a picture you like may be in Dropbox or somewhere else. So there is some benefit to having to choose the app you want to upload the picture from.