we all know that they are to be played simply as 4 semiquaver notes.
"There are various reasons for why it was notated as a grace note. Partly it followed rules of harmonic writing by writing consonant notes as "main" notes and strong dissonance as expressive embellishment
It sounds cool when you do it right, that's why.
PP - you are probably aware that the baroque practice w ornaments - has the grace note starting On the beat, and taking half of the value of the note that follows. .....
As I understand it, the grace note is the one that has a slash through it, like thishttps://www.notation.com/ComposerDocs3/gracenotechart.gifThe appoggiatura we're discussing does not have a slash through it, like thishttps://www.gutenberg.org/files/19499/19499-h/images/fig50-52.jpg
PP - you are probably aware that the baroque practice w ornaments - has the grace note starting On the beat, and taking half of the value of the note that follows.
My teacher once told me the reason, and I think it was the one already cited --- to prevent people from adding ornaments to this kind of appoggiatura
Also it was not uncommon for the notes notated as if they were embellishments to actually be an integral part of the melody.