You explained it nicely, however...
For me as a beginner to intermediate player, it takes too much effort to "move" the chord i see written on the paper/screen by specific number of notes while playing the song at the same time.
This makes it unrealistic to play even simple song without rewriting it to paper and/or practicing few times "unclean" - If i don't play song from memory only, i read C chord, i play C chord, oh no, wait, it should have been ... (counting on fingers) ... "F", because i play in different key now...
This can be problem in group playing - "Hey people, let's play song X in key of C", damn, i have it in G only...
Many experienced players can easily transpose on the fly, but app allows also less seasoned players to join "unexpected" song.
It is not necessary, but convenient helper extending the possibilities...
As i haven't seen any app (suggestions/tips welcome) that would suit my needs, be nicely controllable/appealing, and "offline", I have recently started playing with android development, and am trying to get my own songbook running.
This is however in such early stage, that it will take many weeks before i even ask fellow Ukers here to test/give opinions.