It just strikes me that the point of GCSEs was to get into sixth form and the point of A levels is to get onto the next stage.
However, the school's opinion about likely grades at GCSE became completely obsolete on results day. If the school predicted GCSE grades conservatively, it wouldn't stop you getting a conditional offer and being able to take it up if you get the grades.
In contrast, if the sixth form predicts low for A levels, then there is no way that you'll get conditional offers from the more aspirational universities.
So, the 6th form teachers view can make a material difference to outcomes.
PS this is just theoretical. I'm not fretting about DS. His school are predicting somewhere between A and A* for him.