I don’t know, but I’d hazard a guess that it’s to distinguish between a fricative and/or voiced sound and a non-fricative/non-voiced one. Probably between two of these three:
See also ç in the related languages: Catalan, Occitan, and French.
@davel @FunkyStuff The letter ç has an interesting history. It is a variant of the letter z, not the letter c. The letter z in the Visigothic script had very curly ends: ꝣ.
https://en.wikipedia.org/wiki/Visigothic/_script
Some (Portugguese) place names are also written with ç or z in different languages: Açores / Azores, Moçambique / Mozambique
And here we are in Turkey using it to show "ch" as in "channel" sound.
maybe it's a memory of the time when spanish had 6+ sibilants. or maybe the author still had more than the 2 (or 3) modern spanish has.