Say you've got a bunch of pics in a directory or in your iPhone photo roll and amongst those there's a set of consecutive pics that form a grouping. You might have looked at a unit to rent and taken several pictures of the place - these photos would be in a consecutive sequence, bounded at both ends by photos taken at other times and places.
A photo viewing app might be able to automatically determine such groupings. It could use the photo metadata about when and where the photos were taken, and form groups of photos where each successive photo was not taken too far away from, and not too long after the one prior to it. Perhaps some AI techniques could help out as well (though I'm a bit doubtful of any of the existing techniques being good enough to do a sufficiently useful job).
It's hard to know how well these groupings could be automated without actually trying it. Lets consider the possibility that the software could do a reasonable job.
The iPhone photo roll is just a flat sequence of photos. Each photo you take gets appended to the end of it. As people have pointed out, the iPhone tends to eschew hierarchial storage of information within folders and sub-folders, instead using flat storage like this. Storing information in a flat structure in chronological order seems a reasonable solution that meshes reasonably well with human memory. One problem with it, though is that you can end up with a huge number of items -- in this case photos -- all lumped in together.
An app that could detect the photo groupings could have ways to browse the photos in terms of them.
Perhaps each grouping would be displayed as a line of thumbnails, with the groupings shown one after the other as you go down the screen. You'd scroll down to view more of the groupings. If there were more photos in a grouping than you could fit in a line of the screen (a likely possibility) it could continue the line off to the right - requiring a horizontal scroll to see them. Or maybe it would just only show as many as would fit on the screen, and use an icon to show there were more. You could click on that to view the first unshown photo.
I think it mightn't matter so much if you only see some of the photos in each group - hopefully the ones you would see would be enough to remind you of the subjectmatter of those photos and know if any one you were after was in that group or not. And since only a single line is shown for each group, it means that you could (i'd think) be able to scroll more quickly through all of your photos.
Of course it might not be as efficient if a lot of groupings only contained a one, two or a handful of photos.
There are other possibilities for displaying based on groupings. The screen could show one grouping at a time, where you swipe to the right to see the next group. And if too many photos in the group to display at once, scroll down to see the others (like how he iPhone photo roll works).
The general idea is that using photo groupings would be a way to give a lightweight, automatic structure to the sequence of photos -- without having hierarchies, thus preserving the benefits of 'flat' layouts iPhone uses.
When viewing the photos one by one and you got to the first image in a photo group, it could do the following. Instead of viewing that first photo, it could show thumbnails for all the photos in the group. You click on one of them to jump straight to it. Of if you just scrolled across to the next photo it would _then_ show you the first photo in the group.
Alternatively, when you hit the first photo in the group it could show it, but then underneath it show a line of thumbnails for the other photos in the group (and perhaps it would do the same when you were viewing any of the other photos in the group).
Regarding the automatic determining of the groups, perhaps the software could let you manually adjust its auto-groupings. It could also let you create your own groupings and give the groupings names. I suspect though that for a device like the iPhone, this'd be getting too complicated.