The thing is, you do need some idea of what is expected.
Doesn't have to be a predicted grade
You need an idea in y8 / y9 when selecting options. e.g. If you don't know you are ace at Drama but hopeless at Art you might pick Art over Drama.
Predicted grades in y8/9 are even more nonsensical than at KS4.
You'll know if you're good at a subject from your class marks, feedback, reports etc and generally knowing yourself and your interests.
You need an idea end y10 / early y11 to inform options for 6th form. What schools might accept you, what subjects might be suitable, A levels v BTECs etc.
You'll have current working levels on pieces of classwork. Youll also know your strengths and weaknesses across your subjects. If a student can't say they're good at a subject based on being in school every week then it's probably not an A level for them.
And throughout school you need an idea where, if so minded, you as a parent might like to arrange some extra help.
Again, not covered by a predicted grade. Current working levels will show current performance. If students are getting the same targets day in day out then they either need to act on them or they may benefit from extra help.
A predicted grade shouldn't be the reason to arrange additional help or not. Otherwise we get situations where (for example) student is predicted a 6 but ends up with a 4 and the parent says "but you predicted them a 6". A 6 might have been the best judgement I had based on classwork, but if they've had a bad day or not revised properly I can't change it.
To my mind it isn't acceptable for schools to say 'we have no idea'. Even if it is with a range, schools need to give some clue.
I can list strengths and weaknesses for every child I teach.
If a child is working hard, acting on feedback, preparing for assessments, listening to advice and guidance, completing homework etc then there's no reason they won't reach their potential at the end of y11.
If they aren't doing the above then I'll be on the phone home because their progress is a concern as they're not engaging with the learning. If they could benefit from a little extra support then I'd have called home or put things in place in school.
A predicted grade is largely meaningless as I know nothing about the national cohort that they're going to be norm referenced against.