Whilst this might be true for most of the users, there are edge cases where it’s not.
I have some z-wave sensors which currently only work when configured manually in a *.things file and it provided an easy way to use these sensors which would have not been possible using only the gui.
As long it is only recommended and not enforced. I maintain multiple OH installations and for me it’s easy to share functionality between them by creating the files once and then just share them through cloud sync.
That would not be possible with the gui.