One example is that in fonts containing Vietnamese diacritics I’d like to group them together and not have them between the other diacritics.
But what glyph do you put in that group? There are the obvious ones like
Acircumflexacute but what about
Acircumflex? It is used in more then a dozen languages. So the only way to sort them is by base glyph.
If you like to get an overview during design time, I suggest a list filter in font view. Or use
Languages > Latin > Vietnamese.
I use a whole lot of different custom encodings per font project, and all projects are different. Different encodings to help me with specific tasks in the whole process. Some encodings for design, some for spacing, another for hinting, several for production, and many encodings for customer projects. Surely I handle such encodings in text editors and Excel, but I would not survive missing the ability to drag and sort them visually as well. Nobody can sort shapes of thousands of glyphs by looking at their (friendly) names in a text editor, Rainer.
You can use an old FontLab (demo) in index mode to sort visually, then select all, copy, and paste in a text file to use in Glyphs.