As a user who would also want this feature, I would say it is because you sometimes want to see full detail. If, for example, someone sends you a link to a painting or some old coins you might be buying or whatever.
There's now a link next to the "Download Photo" button called "View Original" when you click to view the larger photos.
Update
Unfortunately on Chrome this link doesn't work because the photo is stored in a way that tells Chrome to automatically download the photo. I'll have to dig in to figure out if there is a way to force the photo to be displayed instead of automatically downloaded.
Overall, it looks great. My family would definitely use it, for similar reasons to yours. Nice work!
You want "Content-disposition: inline" or no content-disposition header.