There are different wifi places your various devices knew wifi codes for. Let's make this knowledge accessible. Consider these situations:
1)You cannot [the owner is not present] or do not want to [time, annoyance] ask 4 key again
2)You have a couch surfer and the wifi (pswd) is on another floor - the easiest way to get your wifi pswd
3)The paper with the wifi password is lost and noone around is technical enough
The app uses a crossplatform vehicle of Dropbox [or Evernote, or Google Keep] so that one does not need to download it to access the paswiords-keys,,,
The fresh app asks to use the user's Dropbox or Evernote or Keep. Then it sniffes the wifi passwiord keys in the device and (at least) when the new ones are being added. It adds the keys to the dbf table [SSID, wifi key and an optional place nickname] and uploads them using the crossplatform vehicle /Dropbox, Evernote or Keep/. It also lets the user to modify the stored key records. The user is able to access them over the internet and sync/download to other devices.
A developer's note /there is a Dropbox API to enable a custom access/
A next generation-version note: The app sits in background of the device. The dbf is upgraded of custom behaviours. App compares the wifis in or out of range with the dbf and triggers the assigned behaviour accordingly.