This app would be based on an idea that not all your photos should be saved into the default Photos app. Because maybe you have some private pics that should not be easily accessible and should not be synced between all your devices via iCloud.
There are "private albums" apps - actually ton of them with a few very good looking but as far as I know they mostly don't offer to take the pictures so you need to import them manually.
With PrivateCam users could take pictures directly in the app and then they would be stored and locked with PIN or Face ID. You could even prevent them appearing in a device backups.
The hardest part would be the picture taking, assuming you want to build custom camera component - there is also the option to ask iOS to take a picture and give you the result.
It also occurred to me that the app could display pictures maybe only in specified locations (via GPS). So for example unless you are home, the app wouldn't even hint that there are pictures.