Scanners are great. Who has a scanner these days? I appreciate that there might be a better solution, but this seems pretty pragmatic. And looking at the results, it also is pretty damn good.
Most of the tech-heavy people I know have pretty much abandoned personal printers, but the non-techies still have and use them, and most of them are using multifunction devices that scan/copy/print.
In my purely anecdotal world, this app is then a good solution for the techies who have a smartphone but no scanner. On the flip side, those techies tend not be the type to collect tons of sentimental photos, so I'm not sure how large the market would be.