Why are many simple appoggiaturas not spelled out?
E.g., a quarter with an 8th note appoggiatura which would give the same as two 8th notes.
One could notate many passages using appoggiaturas but they are not. Hence there must be some distinction between using them and not because in some cases they actually obscure the notation rather than just notating them out directly.
e.g., a descending scale in 8ths could be written using any number of appoggiaturas or just 8ths. The wiki page suggests there is no real difference, but if that is the case then why obscure the notation in some cases but not others.
The only real reason I can think of is that they explicitly distinguish important notes, the principle note. If one plays the piece without the appoggiaturas then it would "work". When notated out this distinction is removed and one has to use theory or ear to know which note is the principle note.