One character can use several glyphs
A base consonant may combine with marks, contextual forms, or positioned components. The final display can involve multiple glyph IDs even though the source text contains a familiar character sequence.
Some glyphs have no Unicode code point
Ligatures, half forms, below-base forms, and reusable components are often unencoded. OpenType substitutions select them during shaping, so an editor must preserve and expose them rather than showing only the cmap.
Glyph IDs are structural
Substitution and positioning tables refer to glyph IDs. Reordering or deleting glyphs can silently break shaping, which is why Font Studio keeps the template order immutable.