Let's say you are scrolling through your photos at dropbox.com/photos looking for something specific. You see a thummbnail that might be what you want, so you click on it. Having seen it full size, it's not what you want, so you click back.
Why does the photos page then go right back to the very top!!! If you've just scrolled your way through 100s or even 1000s of photos, it's really annoying for it to go back to the beginning just because you viewed a photo.
You then have to scroll all the way down again and try to find your place.